{
my $giveMD5 = \&Foo::giveMD5;
my %givenMD5;
*Foo::giveMD5 = sub {
    my $md5 = $givenmd5{@_} ||= $giveMD5->( @_ ); # execute old to obtain "real" MD5
# do evil things to op tree for given parameters if first time
    return $md5; # the "right" value
};
}