Add a Function abstract class and a Function database.

src/org/nwapw/abacus/number/Function.java

@@ -0,0 +1,16 @@
+package org.nwapw.abacus.number;
+
+public abstract class Function {
+
+    public int arity;
+
+    protected abstract Number applyInternal(Number...params);
+
+    public Number apply(Number...params) throws IllegalArgumentException {
+        if(params.length != arity)
+            throw new IllegalArgumentException("Invalid number of arguments: Function takes "
+                    + arity + ", " + params.length + " provided.");
+        return applyInternal(params);
+    }
+
+}

src/org/nwapw/abacus/number/FunctionDatabase.java

@@ -0,0 +1,21 @@
+package org.nwapw.abacus.number;
+
+import java.util.HashMap;
+
+public class FunctionDatabase {
+
+    private HashMap<String, Function> functions;
+
+    private void registerDefault(){
+
+    }
+
+    public FunctionDatabase(){
+        functions = new HashMap<>();
+    }
+
+    public Function getFunction(String name){
+        return functions.get(name);
+    }
+
+}