=begin pod
=TITLE class X::Syntax::NegatedPair
class X::Syntax::NegatedPair does X::Syntax { }
Thrown if a colonpair illegally has a value, for example C<:!a(1)>.
This is an error because the C<!> negation implies that the value is C<False>.
A typical error message from this class is I<Argument not allowed on
negated pair with key 'a'>.
=head1 Methods
=head2 key
Returns the key of the pair that caused the error.
=end pod