esign space,
    ie. the union of all the Boxes in the Region.
    For efficiency, Boxes within a Region should ideally not overlap, but
    functionality is not compromised if they do.

    The minimum and maximum values are expressed in normalized coordinates.

    A Substitution is a dict mapping source glyph names to substitute glyph names.

    Returns data is in similar but different format.  Overlaps of distinct
    substitution Boxes (*not* Regions) are explicitly listed as distinct rules,
    and rules with the same Box merged.  The more specific rules appear earlier
    in the resulting list.  Moreover, instead of just a dictionary of substitutions,
    a list of dictionaries is returned for substitutions corresponding to each
    unique space, with each dictionary being identical to one of the input
    substitution dictionaries.  These dictionaries are not merged to allow data
    sharing when they are converted into font tables.

    Example:
    >>> condSubst = [
    ...     # A list of (Region, Substitution) tuples.
    ...     ([{"wght": (0.5, 1.0)}], {"dollar": "dollar.rvrn"}),
    ...     ([{"wght": (0.5, 1.0)}], {"dollar": "dollar.rvrn"}),
    ...     ([{"wdth": (0.5, 1.0)}], {"cent": "cent.rvrn"}),
    ...     ([{"wght": (0.5, 1.0), "wdth": (-1, 1.0)}], {"dollar": "dollar.rvrn"}),
    ... ]
    >>> from pprint import pprint
    >>> pprint(overlayFeatureVariations(condSubst))
    [({'wdth': (0.5, 1.0), 'wght': (0.5, 1.0)},
      [{'dollar': 'dollar.rvrn'}, {'cent': 'cent.rvrn'}]),
     ({'wdth': (0.5, 1.0)}, [{'cent': 'cent.rvrn'}]),
     ({'wght': (0.5, 1.0)}, [{'dollar': 'dollar.rvrn'}])]
    c