code: function absoluteValue(node) {
    if (!Node.Type.isFunction(node, 'abs')) {
      return Node.Status.noChange(node);
    }
    if (node.args.length > 1) {
      return Node.Status.noChange(node);
    }
    let newNode = clone(node);
    const argument = newNode.args[0];
    if (Node.Type.isConstant(argument, true)) {
      newNode = Node.Creator.constant(math.abs(argument.eval()));
      return Node.Status.nodeChanged(
        ChangeTypes.ABSOLUTE_VALUE, node, newNode);
    }
    else if (Node.Type.isConstantFraction(argument, true)) {
      const newNumerator = Node.Creator.constant(
        math.abs(argument.args[0].eval()));
      const newDenominator =  Node.Creator.constant(
        math.abs(argument.args[1].eval()));
      newNode = Node.Creator.operator('/', [newNumerator, newDenominator]);
      return Node.Status.nodeChanged(
        ChangeTypes.ABSOLUTE_VALUE, node, newNode);
    }
    else {
      return Node.Status.noChange(node);
    }
  }
docs: Evaluates abs() function if it's on a single constant value. Returns a Node.Status object.