sub fails-if-passed-Rat    (FatRat   $r) { say WHAT $r } ;
sub works-if-passed-number (FatRat() $r) { say WHAT $r } ;
fails-if-passed-Rat    1.0 ; # typecheck failure
works-if-passed-number 1.0 ; # coerces 1.0 to a FatRat
my FatRat   $r = 1.0 ;       # typecheck failure
my FatRat() $r = 1.0 ;       # should say "not yet implemented"