Helper shapes

Some shapes are only there to build others. Their positions matter, but they should not end up in the layout. Placing them with place* makes them helper shapes:

place* left at (-6um, 0)    # positioned, but never written

A helper is placed exactly like any shape: other placements can use its anchors, and it can be an operand. It is just not written, on whatever layer it is defined. A place* { } block makes every placement in it a helper. Only shapes can be helpers; cells are always placed with place.

Operands on the result's own layer are already replaced by it, so place* matters most when the result is on another layer, as in the previous lesson, or when a shape only serves as a guide for other placements.