private static Function valueOf(int id) {
            for (Function fnc : values()) {
                if (id == fnc.id) {
                    return fnc;
                }
            }
            return null;
        }
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