Mapping with specified forward and inverse transformations

A TranMap is a Mapping which combines the forward transformation of a
supplied Mapping with the inverse transformation of another supplied Mapping, ignoring
the un-used transformation in each Mapping (indeed the un-used transformation need not
exist).

When the forward transformation of the TranMap is referred to, the transformation actually used is the forward transformation of the first Mapping supplied when the TranMap was constructed. Likewise, when the inverse transformation of the TranMap is referred to, the transformation actually used is the inverse transformation of the second Mapping supplied when the TranMap was constructed.

The TranMap class inherits from the Mapping class.

The TranMap class does not define any new attributes beyond those
which are applicable to all Mappings.

The TranMap class does not
define any new functions beyond those which are applicable to all Mappings.

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