@Override
    protected void onSerialize(Serializer buf) {
        super.onSerialize(buf);
        buf.putByte(null, (byte)fnc.id);
    }
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