These allows for dictionary lookups which fall back to a different,
older keyword. Both lookup and lookup-or-default methods are provided;
lookupBackwardsCompatible and lookupOrDefaultBackwardsCompatible,
respectively.
A list of keywords are provided to these methods. The first is taken to
be the current preferred name of the entry, and subsequent elements are
considered to relate to older syntax. These keywords are tried in turn.
If nothing are found then any errors or messages printed relate to the
first keyword in the list.
This centralises backwards compatability logic and allows for current
backwards compatibility settings to be conveniently searched for.
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