diff --git a/project.clj b/project.clj
index 6aa55f3..1e6a442 100644
--- a/project.clj
+++ b/project.clj
@@ -9,3 +9,5 @@
                  [net.clojars.schneau/psb2 "1.1.0"]]
   :main ^:skip-aot propeller.core
   :repl-options {:init-ns propeller.core})
+  ;:jvm-opts ^:replace [])
+
diff --git a/src/propeller/core.cljc b/src/propeller/core.cljc
index e85cab7..3b906d2 100755
--- a/src/propeller/core.cljc
+++ b/src/propeller/core.cljc
@@ -31,10 +31,15 @@
          :population-size         500
          :max-initial-plushy-size 100
          :step-limit              200
+         ;: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 0.5 :crossover 0.5}
+         ;:variation               {:umad 1.0 :crossover 0.0}
          :elitism                 false
          :PSB2-path               ""
          :PSB2-problem            (clojure.string/replace (first args) #"PSB2." "")}
diff --git a/src/propeller/gp.cljc b/src/propeller/gp.cljc
index 6774a3e..47d8d82 100755
--- a/src/propeller/gp.cljc
+++ b/src/propeller/gp.cljc
@@ -28,8 +28,6 @@
                             :average-genome-length (float (/ (reduce + (map count (map :plushy pop))) (count pop)))
                             :average-total-error   (float (/ (reduce + (map :total-error pop)) (count pop)))})
     (println)))
-;  (clojure.pprint/pprint
-
 
 (defn gp
   "Main GP loop."
diff --git a/src/propeller/problems/PSB2/middle_character.cljc b/src/propeller/problems/PSB2/middle_character.cljc
new file mode 100644
index 0000000..3b3a95c
--- /dev/null
+++ b/src/propeller/problems/PSB2/middle_character.cljc
@@ -0,0 +1,67 @@
+(ns propeller.problems.PSB2.middle-character
+  (: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.tools.metrics :as metrics]))
+
+; ===========  PROBLEM DESCRIPTION  =============================
+; MIDDLE CHARACTER from PSB2
+;Given a string, return the middle
+;character as a string if it is odd length; return the two middle
+;characters as a string if it is even length.
+;
+; Source: https://arxiv.org/pdf/2106.06086.pdf
+; ===============================================================
+
+(defn random-int [] (- (rand-int 201) 100))
+
+(def instructions
+  (utils/not-lazy
+    (concat
+      ;;; stack-specific instructions
+      (get-stack-instructions #{:exec :integer :boolean :char :string :print})
+      ;;; input instructions
+      (list :in1)
+      ;;; close
+      (list 'close)
+      ;;; ERCs (constants)
+      (list "" 0 1 2 random-int))))
+
+(defn error-function
+  ([argmap individual]
+   (error-function argmap individual :train))
+  ([argmap individual subset]
+   (let [program (genome/plushy->push (:plushy individual) argmap)
+         data (get (get argmap :train-and-test-data) subset)
+         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))
+                          :string))
+                      inputs)
+         parsed-outputs (map (fn [output]
+                               (try (read-string output)
+                                    #?(:clj (catch Exception e 1000.0)
+                                       :cljs (catch js/Error. e 1000.0))))
+                             outputs)
+         errors (map (fn [correct-output output]
+                       (if (= output :no-stack-item)
+                         10000
+                         (metrics/levenshtein-distance (str correct-output) (str output))))
+                     correct-outputs
+                     parsed-outputs)]
+     (assoc individual
+       :behaviors parsed-outputs
+       :errors errors
+       :total-error #?(:clj  (apply +' errors)
+                       :cljs (apply + errors))))))
+
+
diff --git a/src/propeller/problems/PSB2/substitution_cipher.cljc b/src/propeller/problems/PSB2/substitution_cipher.cljc
index 83d96d2..9c21bfb 100644
--- a/src/propeller/problems/PSB2/substitution_cipher.cljc
+++ b/src/propeller/problems/PSB2/substitution_cipher.cljc
@@ -41,7 +41,6 @@
       ;;; ERCs (constants)
       (list 0 ""))))
 
-
 (defn error-function
   ([argmap individual]
    (error-function argmap individual :train))
@@ -75,5 +74,4 @@
        :behaviors parsed-outputs
        :errors errors
        :total-error #?(:clj  (apply +' errors)
-                       :cljs (apply + errors))))))
-
+                       :cljs (apply + errors))))))
\ No newline at end of file
diff --git a/src/propeller/tools/metrics.cljc b/src/propeller/tools/metrics.cljc
index bebd992..156265f 100755
--- a/src/propeller/tools/metrics.cljc
+++ b/src/propeller/tools/metrics.cljc
@@ -75,7 +75,7 @@
               ;; we need to initialize the prev-row with the edit distance
               ;; between the various prefixes of b and the empty string
               (range (inc (count (str b))))
-              (str a))))
+              (str a)))))
 
 (defn sequence-similarity
   "Returns a number between 0 and 1, indicating how similar the sequences are