extend
Merge a new L2SemanticValue into an existing L2Synonym. Update the manifest to reflect the merge.
Note that because the L2SemanticValue is new, we don't have to check for existing L2SemanticPrimitiveInvocations becoming synonyms of each other, which is much faster than the general case in mergeExistingSemanticValues.
Parameters
existing
An L2Synonym.
semantic
Another L2SemanticValue representing the same value.