my $x1 = Foo::Number->new()->value( 10 );
my $x2 = Foo::Number->new()->value( 20 );
my $x3 = Foo::Number->new()->value( 30 );
my $x4 = Foo::Number->new()->value( 40 );

my $v1 = Foo::Variable->new()->name( 'a' )->value( 50 );

my $c1 = $v1 - $x1 + $x2 + $x3 - $x4;
my $c2 = $c1 * $v1;