@Override
    public MathFunctionNode clone() {
        MathFunctionNode obj = (MathFunctionNode)super.clone();
        obj.fnc = fnc;
        return obj;
    }
This function is an instruction to negate its argument.

@author baldersheim
@author Simon Thoresen Hult
searchlib/src/main/java/com/yahoo/searchlib/expression/MathFunctionNode.java