Add an abstact class for external functions

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

@@ -0,0 +1,28 @@
+package org.nwapw.abacus.number;
+
+import java.util.HashMap;
+
+public abstract class externalFunction {
+
+    private HashMap<String, Function> functions;
+
+    public externalFunction(){
+        functions=new HashMap<>();
+    }
+
+    public boolean hasFunction(Function x){
+        return functions.containsKey(x);
+    }
+    public Function getFunction(String x){
+        return functions.get(x);
+    }
+    public boolean registerFunction(String x, Function y){
+        if(!functions.containsKey(x))
+            return functions.put(x,y)==null;
+        return false;
+    }
+    public Function functionFor(String x){
+        return null;
+    }
+    public abstract void load();
+}