add trap but R1,R2

Author: gfldex

doc/Language/traps.pod6

@@ -355,6 +355,15 @@ to a power, use parentheses:
     say (-1)²;   # 1
     say (-1)**2; # 1
 
+The operator infix:<but> is narrower then the list constructor. When providing
+a list of roles to mix in, always use parentheses.
+
+    role R1 { method m {} }
+    role R2 { method n {} }
+    my $a = 1 but R1,R2; # R2 is in sink context
+    say $a.^name;
+    # OUTPUT«Int+{R1}␤»
+
 =head1 Subroutine and method calls
 
 Subroutine and method calls can be made using one of two forms: