Property {

    ##[ a <- Int, diff <- Int(range=>[-180,180]) ]##
 
    if    ($diff == 0)         { $tcon->label("zero") }
    elsif (abs($diff) <= 90)   { $tcon->label("1 to 90") }
    elsif (abs($diff) <= 180)  { $tcon->label("91 to 180") }
    elsif (abs($diff) <= 360)  { $tcon->label("181 to 360") }
    else                       { $tcon->label("> 360") }

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

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