The guideline_id attribute is intended to record the
identification of a guideline which caused an ENTRY to be created.
It needs to be available on all ENTRY types.

A guideline system might "propose" any kind of ENTRY: the
collection of a new observation , the prescription of a new drug
or the ordering of a new test, or as you suggest propose a new
diagnosis. Logically it is part of the why (reasoning) but ought
to be differentiated from the composers reasoning (which might
actually be why they do NOT follow the advice as referenced by
the guideline_id!)