=begin pod

=TITLE class X::Syntax::Placeholder

    class X::Syntax::Plaecholder does X::Syntax { }

Syntax error that is thrown when a placeholder is used in a block that
already has a signature attached.

For example

    for 1, 2, 3 -> { say $^x} 

dies with

    ===SORRY!===
    Placeholder variable '$^x' cannot override existing signature

because the C<< -> >> introduces an explicit signature.

=end pod