SYSML17 — Parts is too restrictive

the standard says
""The Sequence diagram describes the flow of control between actors and systems (blocks) or between parts of a system... lifelines into their constituent parts."

But lifelines can be parts, references, parameters, arguments, variables

also think one needs to make the distinction between Actors that are parts of the block and Actors that are external to the block because they can be represented as lifelines as well

also should specify that because an interaction in definition is not tied to a particular block (but the context is derived when executed by a block)... there is no way of telling whether something is a part or a reference at definition time... therefore every head of a lifeline is should as solid and no distinction (like a dotted line for a head) is made in interaction diagrams