Class Variable_Method_Caller

object --+
         |
        Variable_Method_Caller

A class for finding a construction variable on the stack and
calling one of its methods.

We use this to support "construction variables" in our string
eval()s that actually stand in for methods--specifically, use
of "RDirs" in call to _concat that should actually execute the
"TARGET.RDirs" method.  (We used to support this by creating a little
"build dictionary" that mapped RDirs to the method, but this got in
the way of Memoizing construction environments, because we had to
create new environment objects to hold the variables.)

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