package Equation_Problem; 
use Moose;
use MooseX::Invertible_Build;

has mass     => ( isa => Num);
has velocity => ( isa => Num);
has momentum => (
	isa => Num,
	invertible_build => {
		A => 'mass',
		B => 'velocity',
		via => 'A * B',
	},
);

##</code><code>##

package Equation_Problem; 
use Moose;

has mass     => ( isa => Num);
has velocity => ( isa => Num);

sub momentum {
    my $self = shift;
    $self->mass * $self->velocity;
}