diff --git a/src/propeller/problems/PSB2/find_pair.cljc b/src/propeller/problems/PSB2/find_pair.cljc
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+++ b/src/propeller/problems/PSB2/find_pair.cljc
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+(ns propeller.problems.PSB2.find-pair
+  (:require [psb2.core :as psb2]
+            [propeller.genome :as genome]
+            [propeller.push.interpreter :as interpreter]
+            [propeller.problems.data-creation :as dc]
+            [propeller.utils :as utils]
+            [propeller.push.instructions :refer [get-stack-instructions]]
+            [propeller.push.state :as state]
+            [propeller.tools.math :as math]
+            [propeller.gp :as gp]
+            #?(:cljs [cljs.reader :refer [read-string]])))
+
+; ===========  PROBLEM DESCRIPTION  ===============================
+; GCD [GREATEST COMMON DIVISOR] from PSB2
+; Given two integers, return the largest integer that divides each
+; of the integers evenly
+;
+; Source: https://arxiv.org/pdf/2106.06086.pdf
+; ==================================================================
+
+;(def train-and-test-data (psb2/fetch-examples "data" "gcd" 200 2000))
+
+(def train-data (dc/read-data-formatted "find-pair" "train"))
+(def test-data (dc/read-data-formatted "find-pair" "test"))
+
+(defn random-int [] (- (rand-int 201) 100))
+
+(defn map-vals-input
+  "Returns all the input values of a map"
+  [i]
+  (vals (select-keys i [:input1 :input2])))
+
+(defn map-vals-output
+  "Returns the output values of a map"
+  [i]
+  (vals (select-keys i [:output1 :output2])))
+
+(def instructions
+  (utils/not-lazy
+    (concat
+      ;;; stack-specific instructions
+      (get-stack-instructions #{:exec :integer :vector_integer :boolean :string_print :integer_print})
+      ;;; input instructions
+      (list :in1 :in2)
+      ;;; close
+      (list 'close)
+      ;;; ERCs (constants)
+      (list -1 0 1 2 random-int))))
+
+(defn error-function
+  [argmap data individual]
+  (let [program (genome/plushy->push (:plushy individual) argmap)
+        inputs (map (fn [i] (map-vals-input i)) data)
+        correct-outputs (map (fn [i] (map-vals-output i)) data)
+        outputs (map (fn [input]
+                       (state/peek-stack
+                         (interpreter/interpret-program
+                           program
+                           (assoc state/empty-state :input {:in1 (nth input 0)
+                                                            :in2 (nth input 1)})
+                           (:step-limit argmap))
+                         :print))
+                     inputs)
+        parsed-outputs (map (fn [output]
+                              (try (read-string (str "(" output ")"))
+                                   #?(:clj  (catch Exception e 1000000.0)
+                                      :cljs (catch js/Error. e 1000000.0))))
+                            outputs)
+        errors (map (fn [correct-output output]
+                      (if (not= (count output) (count correct-output))
+                        1000000.0
+                        (apply + (map (fn [c o] (if (and (number? c) (number? o))
+                                                  (math/abs (- c o))
+                                                  1000000.0)) correct-output output))))
+                    correct-outputs
+                    parsed-outputs)
+         null (prn {:output (first outputs) :correct-output (first correct-outputs) :parsed (first parsed-outputs) :error (first errors)})]
+    (assoc individual
+      :behaviors parsed-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-data
+       :testing-data            test-data
+       :case-t-size             (count train-data)
+       :ds-parent-rate          0
+       :ds-parent-gens          1
+       :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))))
+  (#?(:clj shutdown-agents)))
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