diff --git a/src/propeller/problems/PSB2/coin_sums.cljc b/src/propeller/problems/PSB2/coin_sums.cljc
new file mode 100644
index 0000000..3c5a73c
--- /dev/null
+++ b/src/propeller/problems/PSB2/coin_sums.cljc
@@ -0,0 +1,96 @@
+(ns propeller.problems.PSB2.coin-sums
+  (:require [psb2.core :as psb2]
+            [propeller.genome :as genome]
+            [propeller.push.interpreter :as interpreter]
+            [propeller.utils :as utils]
+            [propeller.push.utils.helpers :refer [get-stack-instructions]]
+            [propeller.push.state :as state]
+            [propeller.tools.math :as math]
+            [propeller.gp :as gp]
+            [propeller.push.utils.macros :refer [def-instruction
+                                                 generate-instructions]]))
+
+; ===========  PROBLEM DESCRIPTION  ============================
+; COIN SUMS from PSB2
+; Given a number of cents, find the fewest
+; number of US coins (pennies, nickles, dimes, quarters) needed
+; to make that amount, and return the number of each type
+; of coin as a separate output
+;
+; Source: https://arxiv.org/pdf/2106.06086.pdf
+; ===============================================================
+
+(def train-and-test-data (psb2/fetch-examples "data" "coin-sums" 200 2000))
+
+; Random integer between -100 and 100 (from smallest)
+(defn random-int [] (- (rand-int 201) 100))
+
+(def-instruction
+  :output_pennies
+  ^{:stacks [:integer]}
+  (fn [state]
+    (if (empty? (:integer state))
+      state
+      (let [top-int (state/peek-stack :integer state)]
+        ))))
+       ;(stack-assoc top-int :output 0 state))))
+       ;(assoc map key val)
+
+(def instructions
+  (utils/not-lazy
+    (concat
+      ;;; stack-specific instructions
+      (get-stack-instructions #{:exec :integer :boolean :print})
+      ;;; input instructions
+      (list :in1)
+      ;;; close
+      (list 'close)
+      ;;; ERCs (constants)
+      (list 0 1 5 10 25))))
+
+(defn error-function
+  [argmap data individual]
+  (let [program (genome/plushy->push (:plushy individual) argmap)
+        inputs (map (fn [i] (get i :input1)) data)
+        correct-outputs (map (fn [i] (get i :output1)) data)
+        outputs (map (fn [input]
+                       (state/peek-stack
+                         (interpreter/interpret-program
+                           program
+                           (assoc state/empty-state :input {:in1 input})
+                           (:step-limit argmap))
+                         :integer))
+                     inputs)
+        errors (map (fn [correct-output output]
+                      (if (= output :no-stack-item)
+                        1000000
+                        (min 1000.0 (math/abs (- correct-output output)))))
+                    correct-outputs
+                    outputs)]
+    (assoc individual
+      :behaviors outputs
+      :errors errors
+      :total-error #?(:clj  (apply +' errors)
+                      :cljs (apply + errors)))))
+
+(defn -main
+  "Runs propel-gp, giving it a map of arguments."
+  [& args]
+  (gp/gp
+    (merge
+      {:instructions            instructions
+       :error-function          error-function
+       :training-data           (:train train-and-test-data)
+       :testing-data            (:test train-and-test-data)
+       :max-generations         300
+       :population-size         1000
+       :max-initial-plushy-size 250
+       :step-limit              2000
+       :parent-selection        :lexicase
+       :tournament-size         5
+       :umad-rate               0.1
+       :variation               {:umad 1.0 :crossover 0.0}
+       :elitism                 false}
+      (apply hash-map (map #(if (string? %) (read-string %) %) args)))))
+
+
diff --git a/src/propeller/problems/software/number_io.cljc b/src/propeller/problems/software/number_io.cljc
index 2acae61..fcbef5f 100755
--- a/src/propeller/problems/software/number_io.cljc
+++ b/src/propeller/problems/software/number_io.cljc
@@ -71,9 +71,9 @@
                            program
                            (assoc state/empty-state :input {:in1 (first input)
                                                             :in2 (last input)}
-                                                    :output '(""))
+                                                    :print '(""))
                            (:step-limit argmap))
-                         :output))
+                         :print))
                      inputs)
         parsed-outputs (map (fn [output]
                               (try (read-string output)
diff --git a/src/propeller/problems/software/smallest.cljc b/src/propeller/problems/software/smallest.cljc
index a70ac59..9ca4dbb 100755
--- a/src/propeller/problems/software/smallest.cljc
+++ b/src/propeller/problems/software/smallest.cljc
@@ -75,9 +75,9 @@
                                                             :in2 (get input 1)
                                                             :in3 (get input 2)
                                                             :in4 (get input 3)}
-                                                    :output '(""))
+                                                    :print '(""))
                            (:step-limit argmap))
-                         :output))
+                         :print))
                      inputs)
         errors (map (fn [correct-output output]
                       (let [parsed-output (try (read-string output)
diff --git a/src/propeller/push/instructions/input_output.cljc b/src/propeller/push/instructions/input_output.cljc
index 9d52d54..b597c7b 100755
--- a/src/propeller/push/instructions/input_output.cljc
+++ b/src/propeller/push/instructions/input_output.cljc
@@ -31,9 +31,9 @@
   :print_newline
   ^{:stacks [:print]}
   (fn [state]
-    (let [current-output (state/peek-stack state :output)
-          popped-state (state/pop-stack state :output)]
-      (state/push-to-stack popped-state :output (str current-output \newline)))))
+    (let [current-output (state/peek-stack state :print)
+          popped-state (state/pop-stack state :print)]
+      (state/push-to-stack popped-state :print (str current-output \newline)))))
 
 (def _print
   ^{:stacks [:print]
@@ -46,10 +46,10 @@
                                  (char? top-item))
                            top-item
                            (pr-str top-item))
-            current-output (state/peek-stack state :output)
-            popped-state (state/pop-stack (state/pop-stack state stack) :output)]
+            current-output (state/peek-stack state :print)
+            popped-state (state/pop-stack (state/pop-stack state stack) :print)]
         (state/push-to-stack popped-state
-                             :output
+                             :print
                              (str current-output top-item-str))))))
 
 (generate-instructions
diff --git a/src/propeller/push/state.cljc b/src/propeller/push/state.cljc
index b49227e..647509c 100755
--- a/src/propeller/push/state.cljc
+++ b/src/propeller/push/state.cljc
@@ -7,8 +7,9 @@
                       :exec           '()
                       :float          '()
                       :input          {}
+                      :output         {}
                       :integer        '()
-                      :output         '("")
+                      :print          '("")
                       :string         '()
                       :vector_boolean '()
                       :vector_float   '()