Unifies the Span information from the specified
left and
right expressions, and applies it to the specified
to expression. Unification proceeds as follows: - If neither
left nor
right have Span information, no changes are made - If one of
left or
right has Span information, that single Span information entry is removed from the Span mapping dictionary and used to create a new entry that maps from the
expression to the Span information. - If both left and right have Span information, both entries are removed from the Span mapping dictionary, a new Span is created that contains the union of the original Spans, and a new entry is added to the dictionary that maps from to expression to this new Span.