Property {

    ##[ a <- Int, n <- Int, diff <- Int(range=>[-180,180]) ]##
 
    my $b = $a + 360*$n + $diff;
    my $delta = abs($b - $a);

    if    ($delta == 0)    { $tcon->label("zero") }
    elsif ($delta <= 90)   { $tcon->label("1 to 90") }
    elsif ($delta <= 180)  { $tcon->label("91 to 180") }
    elsif ($delta <= 360)  { $tcon->label("181 to 360") }
    else                   { $tcon->label("> 360") }

    angdiff($a, $b) == abs($diff)

}, name => "angdiff holds to defn of angular difference";