Abstract the call functionality, and add TreeValueFunctionNode.

core/src/main/java/org/nwapw/abacus/parsing/ShuntingYardParser.java

@@ -144,7 +144,7 @@ public class ShuntingYardParser implements Parser<Match<TokenType>>, PluginListe
             while (!matches.isEmpty() && matches.get(0).getType() != TokenType.INTERNAL_FUNCTION_END) {
                 TreeNode argument = constructRecursive(matches);
                 if (argument == null) return null;
-                node.prependChild(argument);
+                node.getChildren().add(0, argument);
             }
             if (matches.isEmpty()) return null;
             matches.remove(0);

core/src/main/java/org/nwapw/abacus/tree/NumberReducer.java

@@ -46,7 +46,7 @@ public class NumberReducer implements Reducer<NumberInterface> {
             for (int i = 0; i < convertedChildren.length; i++) {
                 convertedChildren[i] = (NumberInterface) children[i];
             }
-            Function function = abacus.getPluginManager().functionFor(((FunctionNode) node).getFunction());
+            Function function = abacus.getPluginManager().functionFor(((FunctionNode) node).getCallTo());
             if (function == null) return null;
             return function.apply(convertedChildren);
         }