file.p6

use v6;

class Replacement {

}

use Lib Str;

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

dd $g ~ "abc";
dd "abc" ~ $g;

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) {...}
}

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

		method new($s) {
			self.bless(attr=>$s.subst('&', '&amp;').subst('<', '&lt'));
		}
	}

	multi sub infix:<~>(Example \l, T \r){ note "op1"; };
	multi sub infix:<~>(T \l, Example \r){ note "op2"; };
	
	{
	    Guard => Example,
	    '&infix:<~>' => &infix:<~>,
#		'&infix:<~>' => sub (T \l, Example \r){ l ~ r },
	}
}