file.p6

use v6;

class Replacement {

}

use Lib Str;

my $g = Guard.new("");

dd $g ~ "abc";

Lib.pm

use v6;

role ConcatGuard[::T-right] {
	has T-right $.attr is rw; 
	method left-type () is nodal { self.WHAT }
	method right-type () is nodal { T-right }
	method convert () {...} # --> T-right
	method new (T-right) {...}
	# our proto sub infix:<~>(|) {*}
	# our multi sub infix:<~>(T-left \l, T-right \r) {
	# 	l.convert()
	# }
	# our multi sub infix:<~>(T-right \l, T-left \r) {
	# 	r.new(l);
	# }
}

sub EXPORT($T = Str) {
	class Example is Mu does ConcatGuard[$T] { 
		method convert(){
			$.attr.subst('&lt;','<').subst('&amp;', '&');
		}

		method new($s) {
			self.bless(attr=>$s.subst('&', '&amp;').subst('<', '&lt'));
		}
	}
	
	dd $T.WHAT;
	constant T := $T;
	dd T.WHAT;
	{
	    Guard => Example,
	    '&infix:<~>' => sub (Example \l, T \r){ l ~ r },
		'&infix:<~>' => sub (T \l, Example \r){ l ~ r }
	}
}