=over
=item cos EXPR
X X X X
=item cos
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
takes the cosine of C<$_>.
For the inverse cosine operation, you may use the C
function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
=back