equivalent
Given an L2SemanticValue, see if there's already an equivalent one in this manifest. If an L2SemanticPrimitiveInvocation is supplied, look for a recursively synonymous one.
Answer the extant L2SemanticValue if found, otherwise answer null. Note that there may be multiple L2SemanticPrimitiveInvocations that are equivalent, in which case an arbitrary (and not necessarily stable) one is chosen.
Return
An L2SemanticValue from this manifest which is equivalent to the given one, or null if no such value is in the manifest.
Parameters
semantic
The L2SemanticValue to look up.