Class DefaultEnvironmentCall

object --+
         |
        DefaultEnvironmentCall

A class that implements "global function" calls of
Environment methods by fetching the specified method from the
DefaultEnvironment's class.  Note that this uses an intermediate
proxy class instead of calling the DefaultEnvironment method
directly so that the proxy can override the subst() method and
thereby prevent expansion of construction variables (since from
the user's point of view this was called as a global function,
with no associated construction environment).

x.__init__(...) initializes x; see x.__class__.__doc__ for signature