optional and conditional blocks

For Vec2D and Vec3D there is a method set_mag which can accept an optional block, and if given it should evaluate to a boolean. The magnitude is only set when the block evaluates to true (without a block the magnitude is set unconditionally providing new value is a scalar).

See below for a Vec2D example where velocity is an instance of Vec2D:-

velocity.set_mag(MAXSPEED) { velocity.mag > MAXSPEED }

In this way we can limit the velocity of a particle say, to a maximum speed.