Make applicable into an interface.

core/src/main/kotlin/org/nwapw/abacus/function/NumberFunction.kt

@@ -8,4 +8,4 @@ import org.nwapw.abacus.number.NumberInterface
  * This function takes some number of input NumberInterfaces and returns
  * another NumberInterface as a result.
  */
-abstract class NumberFunction : Applicable<NumberInterface, NumberInterface>()
+abstract class NumberFunction : Applicable<NumberInterface, NumberInterface>

core/src/main/kotlin/org/nwapw/abacus/function/NumberOperator.kt

@@ -12,4 +12,5 @@ import org.nwapw.abacus.number.NumberInterface
  */
 abstract class NumberOperator(associativity: OperatorAssociativity, type: OperatorType,
                               precedence: Int) :
-        Operator<NumberInterface, NumberInterface>(associativity, type, precedence)
+        Operator(associativity, type, precedence),
+        Applicable<NumberInterface, NumberInterface>

core/src/main/kotlin/org/nwapw/abacus/function/Operator.kt

@@ -9,5 +9,5 @@ package org.nwapw.abacus.function
  * @param type the type of this operator, used for parsing (infix / prefix / postfix and binary / unary)
  * @param precedence the precedence of this operator, used for order of operations.
  */
-abstract class Operator<T: Any, O: Any>(val associativity: OperatorAssociativity, val type: OperatorType,
-                                               val precedence: Int) : Applicable<T, O>()
+open class Operator(val associativity: OperatorAssociativity, val type: OperatorType,
+                                               val precedence: Int)

core/src/main/kotlin/org/nwapw/abacus/function/TreeValueFunction.kt

@@ -9,4 +9,4 @@ import org.nwapw.abacus.tree.TreeNode
  * A function that operates on parse tree nodes instead of on already simplified numbers.
  * Despite this, it returns a number, not a tree.
  */
-abstract class TreeValueFunction : ReducerApplicable<TreeNode, NumberInterface, NumberInterface>()
+abstract class TreeValueFunction : ReducerApplicable<TreeNode, NumberInterface, NumberInterface>