Compilation error due to passing an argument to a negated colonpair
class X::Syntax::NegatedPair does X::Syntax { }
Thrown if a colonpair illegally has a value, for example :!a(1). This is an error because the ! negation implies that the value is False.
A typical error message from this class is Argument not allowed on negated pair with key 'a'.
Returns the key of the pair that caused the error.