Handle Common Drafting Primitives

When the DRAG command is used on a specified drafting primitive, this causes its drafting points to move. Other primitives that use the same drafting points also change position. (Straights and Arcs can also change in shape.) These primitives are the common drafting primitives for specified primitive.

In order to help predict the effect of a DRAG command, the common drafting primitives can be highlighted or queried. The basic syntax is:

HIGhlight [prim_id] COMmon

Query COMmon- at a primitive

where prim_id identifies a drafting primitive. If prim_id is omitted, the common primitives for the current element are highlighted/queried.

The DRAG command can also be used on an individual node of a Straight or Arc. The common primitives of a node can be highlighted or queried:

HIGhlight prim_id COMmon  prim_id

For a Straight these are FPT (‘from’ point), TPT (‘to’ point) or MPT (midpoint); and for an Arc these are FPT, TPT, CPT (centre point) or THPT (‘through’ point).

The node must be currently used by the Straight or Arc. A Straight has a single node, MPT or two nodes, FPT and TPT. An Arc has an FPT, TPT and either a CPT or a THPT. If an unset node or an illegal node is specified, then an error is output.