diff --git a/src/propeller/core.clj b/src/propeller/core.clj
index 342c8a7..c25aa43 100644
--- a/src/propeller/core.clj
+++ b/src/propeller/core.clj
@@ -1,9 +1,9 @@
 (ns propeller.core
   (:gen-class)
-  (:require [propeller.push.instructions boolean char code input-output numeric random string])
-  (:use propeller.gp
-        propeller.push.instructions
-        [propeller.problems simple-regression string-classification]))
+  (:require [propeller.gp :refer :all]
+            [propeller.push.core :refer :all]
+            (propeller.problems [simple-regression :refer :all]
+                                [string-classification :refer :all])))
 
 (defn -main
   "Runs propel-gp, giving it a map of arguments."
diff --git a/src/propeller/genome.clj b/src/propeller/genome.clj
index ae778e1..2d8c971 100644
--- a/src/propeller/genome.clj
+++ b/src/propeller/genome.clj
@@ -1,5 +1,5 @@
 (ns propeller.genome
-  (:use propeller.push.instructions))
+  (:require [propeller.push.core :refer :all]))
 
 (defn plushy->push
   "Returns the Push program expressed by the given plushy representation."
diff --git a/src/propeller/gp.clj b/src/propeller/gp.clj
index 978b3ff..02673fc 100644
--- a/src/propeller/gp.clj
+++ b/src/propeller/gp.clj
@@ -1,5 +1,7 @@
 (ns propeller.gp
-  (:use [propeller genome variation]))
+  (:require [propeller.push.core :refer [instruction-table]]
+            (propeller [genome :refer :all]
+                       [variation :refer :all])))
 
 (defn report
   "Reports information each generation."
@@ -24,7 +26,15 @@
   [{:keys [population-size max-generations error-function instructions
            max-initial-plushy-size]
     :as   argmap}]
-  (println "Starting GP with args:" argmap)
+  ;;
+  (println "Starting GP with args: " argmap)
+  ;;
+  (do (print "Registering instructions... ")
+      (require '[propeller.push.instructions boolean char code input-output
+                 numeric polymorphic string])
+      (println "Done. Registered instructions:")
+      (println (sort (keys @instruction-table))))
+  ;;
   (loop [generation 0
          population (repeatedly
                       population-size
diff --git a/src/propeller/problems/string_classification.clj b/src/propeller/problems/string_classification.clj
index 6929ec0..3434cfa 100644
--- a/src/propeller/problems/string_classification.clj
+++ b/src/propeller/problems/string_classification.clj
@@ -1,6 +1,7 @@
 (ns propeller.problems.string-classification
-  (:use propeller.genome
-        [propeller.push state interpreter]))
+  (:require [propeller.genome :refer :all]
+            (propeller.push [state :refer :all]
+                            [interpreter :refer :all])))
 
 ;; =============================================================================
 ;; String classification
diff --git a/src/propeller/push/instructions.clj b/src/propeller/push/core.clj
similarity index 97%
rename from src/propeller/push/instructions.clj
rename to src/propeller/push/core.clj
index 942d4e8..aad92c2 100644
--- a/src/propeller/push/instructions.clj
+++ b/src/propeller/push/core.clj
@@ -1,4 +1,4 @@
-(ns propeller.push.instructions
+(ns propeller.push.core
   (:require [propeller.push.state :refer [get-args-from-stacks
                                           push-to-stack]]))
 
diff --git a/src/propeller/push/instructions/boolean.clj b/src/propeller/push/instructions/boolean.clj
index 46eb44d..cec31ad 100644
--- a/src/propeller/push/instructions/boolean.clj
+++ b/src/propeller/push/instructions/boolean.clj
@@ -1,12 +1,10 @@
 (ns propeller.push.instructions.boolean
-  (:require [propeller.push.instructions :refer [def-instruction]]
+  (:require [propeller.push.core :refer [def-instruction]]
             [propeller.push.utils :refer [make-instruction]]))
 
-;; Pushes TRUE if the top two BOOLEANs are equal, and FALSE otherwise
-(def-instruction
-  :boolean_=
-  (fn [state]
-    (make-instruction state = [:boolean :boolean] :boolean)))
+;; =============================================================================
+;; BOOLEAN Instructions
+;; =============================================================================
 
 ;; Pushes the logical AND of the top two BOOLEANs
 (def-instruction
diff --git a/src/propeller/push/instructions/char.clj b/src/propeller/push/instructions/char.clj
index ddfaa35..ece1d80 100644
--- a/src/propeller/push/instructions/char.clj
+++ b/src/propeller/push/instructions/char.clj
@@ -1,8 +1,12 @@
 (ns propeller.push.instructions.char
-  (:require [propeller.push.instructions :refer [def-instruction]]
+  (:require [propeller.push.core :refer [def-instruction]]
             [propeller.push.utils :refer [make-instruction]]
             [tools.character :as char]))
 
+;; =============================================================================
+;; CHAR Instructions
+;; =============================================================================
+
 ;; Pushes TRUE onto the BOOLEAN stack if the popped character is a letter
 (def-instruction
   :char_isletter
@@ -22,13 +26,14 @@
   (fn [state]
     (make-instruction state char/is-whitespace [:char] :boolean)))
 
-;; Pops the STRING stack and pushes the top element's constituent characters
-;; onto the CHAR stack, in order. For instance, "hello" will result in the
-;; top of the CHAR stack being o l l e h
+;; Pops the FLOAT stack, converts the top item to a whole number, and pushes
+;; its corresponding ASCII value onto the CHAR stack. Whole numbers larger than
+;; 128 will be reduced modulo 128. For instance, 248.45 will result in x being
+;; pushed.
 (def-instruction
-  :char_allfromstring
+  :char_fromfloat
   (fn [state]
-    (make-instruction state #(map char %) [:string] :char)))
+    (make-instruction state #(char (mod (long %) 128)) [:float] :char)))
 
 ;; Pops the INTEGER stack and pushes the top element's corresponding ASCII
 ;; value onto the CHAR stack. Integers larger than 128 will be reduced modulo
@@ -38,11 +43,10 @@
   (fn [state]
     (make-instruction state #(char (mod % 128)) [:integer] :char)))
 
-;; Pops the FLOAT stack, converts the top item to a whole number, and pushes
-;; its corresponding ASCII value onto the CHAR stack. Whole numbers larger than
-;; 128 will be reduced modulo 128. For instance, 248.45 will result in x being
-;; pushed.
+;; Pops the STRING stack and pushes the top element's constituent characters
+;; onto the CHAR stack, in order. For instance, "hello" will result in the
+;; top of the CHAR stack being o l l e h
 (def-instruction
-  :char_fromfloat
+  :char_allfromstring
   (fn [state]
-    (make-instruction state #(char (mod (long %) 128)) [:float] :char)))
+    (make-instruction state #(map char %) [:string] :char)))
diff --git a/src/propeller/push/instructions/code.clj b/src/propeller/push/instructions/code.clj
index 1c6aeb8..86b4f32 100644
--- a/src/propeller/push/instructions/code.clj
+++ b/src/propeller/push/instructions/code.clj
@@ -1,8 +1,12 @@
 (ns propeller.push.instructions.code
-  (:require [propeller.push.instructions :refer [def-instruction]]
+  (:require [propeller.push.core :refer [def-instruction]]
             [propeller.push.state :as state]
             [propeller.push.utils :refer [make-instruction]]))
 
+;; =============================================================================
+;; CODE and EXEC Instructions
+;; =============================================================================
+
 (def-instruction
   :exec_dup
   (fn [state]
diff --git a/src/propeller/push/instructions/input_output.clj b/src/propeller/push/instructions/input_output.clj
index 2271a0d..5544f07 100644
--- a/src/propeller/push/instructions/input_output.clj
+++ b/src/propeller/push/instructions/input_output.clj
@@ -1,6 +1,10 @@
 (ns propeller.push.instructions.input-output
   (:require [propeller.push.state :as state]
-            [propeller.push.instructions :refer [def-instruction]]))
+            [propeller.push.core :refer [def-instruction]]))
+
+;; =============================================================================
+;; INPUT and OUTPUT Instructions
+;; =============================================================================
 
 ;; Pushes the input labeled :in1 on the inputs map onto the :exec stack
 (def-instruction
diff --git a/src/propeller/push/instructions/numeric.clj b/src/propeller/push/instructions/numeric.clj
index d534569..cd7db7c 100644
--- a/src/propeller/push/instructions/numeric.clj
+++ b/src/propeller/push/instructions/numeric.clj
@@ -1,31 +1,37 @@
 (ns propeller.push.instructions.numeric
-  (:require [propeller.push.instructions :refer [def-instruction]]
+  (:require [propeller.push.core :refer [def-instruction]]
             [propeller.push.utils :refer [generate-functions
                                           make-instruction]]
             [tools.math :as math]))
 
 ;; =============================================================================
-;; FLOAT and INTEGER (polymorphic)
+;; FLOAT and INTEGER Instructions (polymorphic)
 ;; =============================================================================
 
-;; Pushes TRUE onto the BOOLEAN stack if the top two items are equal, and
-;; FALSE otherwise
-(defn- _=
-  [stack state]
-  (make-instruction state = [stack stack] :boolean))
-
 ;; Pushes TRUE onto the BOOLEAN stack if the second item is greater than the top
 ;; item, and FALSE otherwise
 (defn- _>
   [stack state]
   (make-instruction state > [stack stack] :boolean))
 
+;; Pushes TRUE onto the BOOLEAN stack if the second item is greater than or
+;; equal to the top item, and FALSE otherwise
+(defn- _>=
+  [stack state]
+  (make-instruction state >= [stack stack] :boolean))
+
 ;; Pushes TRUE onto the BOOLEAN stack if the second item is less than the top
 ;; item, and FALSE otherwise
 (defn- _<
   [stack state]
   (make-instruction state < [stack stack] :boolean))
 
+;; Pushes TRUE onto the BOOLEAN stack if the second item is less than or equal
+;; to the top item, and FALSE otherwise
+(defn- _<=
+  [stack state]
+  (make-instruction state <= [stack stack] :boolean))
+
 ;; Pushes the sum of the top two items onto the same stack
 (defn- _+
   [stack state]
@@ -48,7 +54,7 @@
   [stack state]
   (make-instruction state
                     #(if (zero? %2)
-                       (list %1 %2) ; push both items back
+                       (list %1 %2) ; push both items back (NOOP)
                        (quot %1 %2))
                     [stack stack]
                     stack))
@@ -61,7 +67,7 @@
   [stack state]
   (make-instruction state
                     #(if (zero? %2)
-                       (list %1 %2) ; push both items back
+                       (list %1 %2) ; push both items back (NOOP)
                        (mod %1 %2))
                     [stack stack]
                     stack))
@@ -80,21 +86,43 @@
 (defn- _fromboolean
   [stack state]
   (make-instruction state
-                    #((if (= stack :float) float int) (if % 1 0))
+                    #((if (= stack :integer) int float) (if % 1 0))
                     [:boolean]
                     stack))
 
-;; Automate type-specific function generation. All resulting functions take a
-;; Push state as their only argument. For FLOAT and INTEGER, create one of each
-;; of the 11 following type-specific functions: =, >, <, +, -, *, QUOT, %, MAX,
-;; MIN, and FROMBOOLEAN. (22 functions total, with syntax e.g. integer_=,
-;; float_min etc.)
+;; Pushes the ASCII value of the top CHAR
+(defn- _fromchar
+  [stack state]
+  (make-instruction state (if (= stack :integer) int float) [:char] stack))
+
+;; Pushes the value of the top STRING, if it can be parsed as a number.
+;; Otherwise, acts as a NOOP
+(defn- _fromstring
+  [stack state]
+  (make-instruction state
+                    #(try ((if (= stack :integer) int float) (read-string %))
+                          (catch Exception e))
+                    [:string]
+                    stack))
+
+;; Pushes the increment (i.e. +1) of the top item of the stack
+(defn- _inc
+  [stack state]
+  (make-instruction state inc [stack] stack))
+
+;; Pushes the decrement (i.e. -1) of the top item of the stack
+(defn- _dec
+  [stack state]
+  (make-instruction state dec [stack] stack))
+
+;; 2 types x 16 functions = 32 instructions
 (generate-functions
   [:float :integer]
-  [_=, _>, _<, _+, _-, _*, _quot, _%, _max, _min, _fromboolean])
+  [_> _>= _< _<= _+ _- _* _quot _% _max _min _inc _dec
+   _fromboolean _fromchar _fromstring])
 
 ;; =============================================================================
-;; FLOAT only
+;; FLOAT Instructions only
 ;; =============================================================================
 
 ;; Pushes the cosine of the top FLOAT
@@ -115,20 +143,14 @@
   (fn [state]
     (make-instruction state math/tan [:float] :float)))
 
-;; Pushes the tangent of the top FLOAT
-(def-instruction
-  :float_tan
-  (fn [state]
-    (make-instruction state math/tan [:float] :float)))
-
-;; Pushes a floating point version of the top INTEGER
+;; Pushes the floating point version of the top INTEGER
 (def-instruction
   :float_frominteger
   (fn [state]
-    (make-instruction state math/tan [:float] :float)))
+    (make-instruction state float [:integer] :float)))
 
 ;; =============================================================================
-;; INTEGER only
+;; INTEGER Instructions only
 ;; =============================================================================
 
 ;; Pushes the result of truncating the top FLOAT towards negative infinity
diff --git a/src/propeller/push/instructions/polymorphic.clj b/src/propeller/push/instructions/polymorphic.clj
new file mode 100644
index 0000000..999a315
--- /dev/null
+++ b/src/propeller/push/instructions/polymorphic.clj
@@ -0,0 +1,110 @@
+(ns propeller.push.instructions.polymorphic
+  (:require [propeller.push.core :refer [def-instruction]]
+            [propeller.push.state :refer [empty-stack?
+                                          peek-stack
+                                          pop-stack
+                                          push-to-stack]]
+            [propeller.push.utils :refer [generate-functions
+                                          make-instruction]]))
+
+;; =============================================================================
+;; Polymorphic Instructions
+;;
+;; (for all types, with the exception of non-data  stacks like auxiliary, tag,
+;; input, and output)
+;; =============================================================================
+
+;; Pushes TRUE onto the BOOLEAN stack if the top two items are equal.
+;; Otherwise FALSE
+(defn- _=
+  [stack state]
+  (make-instruction state = [stack stack] :boolean))
+
+;; Duplicates the top item of the stack. Does not pop its argument (since that
+;; would negate the effect of the duplication)
+(defn- _dup
+  [stack state]
+  (let [top-item (peek-stack state stack)]
+    (if (empty-stack? state stack)
+      state
+      (push-to-stack state stack top-item))))
+
+;; Duplicates n copies of the top item (i.e leaves n copies there). Does not pop
+;; its argument (since that would negate the effect of the duplication). The
+;; number n is determined by the top INTEGER. For n = 0, equivalent to POP.
+;; For n = 1, equivalent to NOOP. For n = 2, equivalent to DUP. Negative values
+;; of n are treated as 0.
+(defn- _duptimes
+  [stack state]
+  (if (or (and (= stack :integer)
+               (>= (count (:integer state)) 2))
+          (and (not= stack :integer)
+               (not (empty-stack? state :integer))
+               (not (empty-stack? state stack))))
+    (let [n (peek-stack state :integer)
+          item-to-duplicate (peek-stack state stack)]
+      nil)
+    state))
+
+
+(defn- _dupitems
+  [stack state]
+  ())
+
+;; Pushes TRUE onto the BOOLEAN stack if the stack is empty. Otherwise FALSE
+(defn- _empty
+  [stack state]
+  (push-to-stack state :boolean (empty-stack? state stack)))
+
+;; Empties the given stack
+(defn- _flush
+  [stack state]
+  ())
+
+;; Pops the given stack
+(defn- _pop
+  [stack state]
+  (pop-stack state stack))
+
+;; Rotates the top three items on the stack (i.e. pulls the third item out and
+;; pushes it on top). Equivalent to (yank state stack-type 2)
+(defn- _rot
+  [stack state]
+  ())
+
+;; Inserts the top item deeper into the stack, using the top INTEGER to
+;; determine how deep
+(defn- _shove
+  [stack state]
+  ())
+
+;; Pushes the given stack's depth onto the INTEGER stack
+(defn- _stackdepth
+  [stack state]
+  ())
+
+;; Swaps the top two items on the stack
+(defn- _swap
+  [stack state]
+  ())
+
+;; Removes an indexed item from deep in the stack. The top INTEGER is used to
+;; determine how deep to yank from
+(defn- _yank
+  [stack state]
+  ())
+
+;; Pushes a copy of an indexed item deep in the stack, without removing it.
+;; The top INTEGER is used to determine how deep to yankdup from
+(defn- _yankdup
+  [stack state]
+  ())
+
+;; 5 types x 1 function = 5 instructions
+(generate-functions [:boolean :char :float :integer :string] [_=])
+
+;; 8 types x 12 function = 96 instructions
+(generate-functions
+  [:boolean :char :code :exec :float :integer :string :zip]
+  [_dup _duptimes _dupitems _empty _flush _pop _rot _shove _stackdepth
+   _swap _yank _yankdup])
diff --git a/src/propeller/push/instructions/random.clj b/src/propeller/push/instructions/random.clj
deleted file mode 100644
index b30a21a..0000000
--- a/src/propeller/push/instructions/random.clj
+++ /dev/null
@@ -1,17 +0,0 @@
-(ns propeller.push.instructions.random
-  (:require [propeller.push.instructions :refer [def-instruction]]))
-
-;;; Pushes a random BOOLEAN
-;(def-instruction
-;  :boolean_rand
-;  (fn [state]
-;    (make-instruction state #(rand-nth [true false]) [] :boolean)))
-;
-;(defn- _rand
-;  "For an INTEGER stack type, pushes a newly generated random INTEGER that is
-;  greater than or equal to MIN-RANDOM-INTEGER and less than or equal to
-;  MAX-RANDOM-INTEGER. Analogous for a FLOAT stack type, with its corresponding
-;  MAX-RANDOM-FLOAT and MIN-RANDOM-FLOAT."
-;  [state stack-type]
-;  (let [data-type (keyword stack-type)]
-;    ()))
diff --git a/src/propeller/push/instructions/string.clj b/src/propeller/push/instructions/string.clj
index 1b5fba5..789a2f6 100644
--- a/src/propeller/push/instructions/string.clj
+++ b/src/propeller/push/instructions/string.clj
@@ -1,7 +1,11 @@
 (ns propeller.push.instructions.string
-  (:require [propeller.push.instructions :refer [def-instruction]]
+  (:require [propeller.push.core :refer [def-instruction]]
             [propeller.push.utils :refer [make-instruction]]))
 
+;; =============================================================================
+;; STRING Instructions
+;; =============================================================================
+
 (def-instruction
   :string_=
   (fn [state]
diff --git a/src/propeller/push/interpreter.clj b/src/propeller/push/interpreter.clj
index 7a6e616..82e223a 100644
--- a/src/propeller/push/interpreter.clj
+++ b/src/propeller/push/interpreter.clj
@@ -1,5 +1,5 @@
 (ns propeller.push.interpreter
-  (:require [propeller.push.instructions :refer [instruction-table]])
+  (:require [propeller.push.core :refer [instruction-table]])
   (:require [propeller.push.state :refer :all]))
 
 (defn interpret-one-step
diff --git a/src/propeller/push/state.clj b/src/propeller/push/state.clj
index 49bf304..03fb788 100644
--- a/src/propeller/push/state.clj
+++ b/src/propeller/push/state.clj
@@ -58,7 +58,9 @@
 (defn push-to-stack
   "Pushes item(s) onto stack."
   [state stack items]
-  (update state stack (if (seq? items) into conj) items))
+  (let [items-list (if (coll? items) items (list items))
+        items-list-no-nil (filter #(not (nil? %)) items-list)]               ; do not push nil items
+    (update state stack into items-list-no-nil)))
 
 (defn get-args-from-stacks
   "Takes a state and a collection of stacks to take args from. If there are
diff --git a/src/propeller/push/utils.clj b/src/propeller/push/utils.clj
index 3ba9a60..793109c 100644
--- a/src/propeller/push/utils.clj
+++ b/src/propeller/push/utils.clj
@@ -1,6 +1,6 @@
 (ns propeller.push.utils
-  (:require [propeller.push.instructions :refer [def-instruction]]
-            [propeller.push.state :as push-state]))
+  (:require [propeller.push.core :refer [def-instruction]]
+            [propeller.push.state :refer :all]))
 
 ;; A utility function for making Push instructions. Takes a state, a function
 ;; to apply to the args, the stacks to take the args from, and the stack to
@@ -8,12 +8,12 @@
 ;; given stacks), and pushes the result onto the return-stack
 (defn make-instruction
   [state function arg-stacks return-stack]
-  (let [popped-args (push-state/get-args-from-stacks state arg-stacks)]
+  (let [popped-args (get-args-from-stacks state arg-stacks)]
     (if (= popped-args :not-enough-args)
       state
       (let [result (apply function (:args popped-args))
             new-state (:state popped-args)]
-        (push-state/push-to-stack new-state return-stack result)))))
+        (push-to-stack new-state return-stack result)))))
 
 ;; Given a sequence of stacks, e.g. [:float :integer], and a sequence of suffix
 ;; function strings, e.g. [_+, _*, _=], automates the generation of all possible
@@ -28,7 +28,7 @@
 ;; Pretty-prints a Push state, for logging or debugging purposes
 (defn print-state
   [state]
-  (doseq [stack push-state/stacks]
+  (doseq [stack stacks]
     (printf "%-15s = " stack)
     (prn (if (get state stack) (get state stack) '()))
     (flush)))
diff --git a/src/propeller/session.clj b/src/propeller/session.clj
index e827cc8..7fca02f 100644
--- a/src/propeller/session.clj
+++ b/src/propeller/session.clj
@@ -1,7 +1,13 @@
 (ns propeller.session
-  (:use [propeller core gp variation selection genome]
-        [propeller.push interpreter instructions state]
-        [propeller.problems simple-regression string-classification]))
+  (:require (propeller [gp :refer :all]
+                       [variation :refer :all]
+                       [selection :refer :all]
+                       [genome :refer :all])
+            (propeller.push [interpreter :refer :all]
+                            [core :refer :all]
+                            [state :refer :all])
+            (propeller.problems [simple-regression :refer :all]
+                                [string-classification :refer :all])))
 
 #_(interpret-program '(1 2 integer_+) empty-push-state 1000)