Semantic Unification is determined by a combination of ACL/outer and inner languages and message semantics.
Not simple variable unification or even syntax.

"Easy" Cases:

Query	Reply
ASK[ P(a,?x)]	REPLY[ P(a,b)]
ASK [id, P(a)]	REPLY [id, Q(a)
TRACE[ (prop=P, obj=O)]	NOTICE[ P(o,a)]

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Exceptions:
Volunteered Information

DEF[ Goal(a, b)]

NOTICE[ Invalid(a,b)]

Difference is expectation of message response:
Model of message semantics also needed to identify query/replies.
Inexact Replies

ASK[ P(a,?x)]

REPLY[ Q(a,b)]

Seems obvious but can be problematic.

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cjp