def expand_varref(varref, extra_vars = nil)
      vars = if extra_vars.nil?
               @varset
             else
               @varset.merge(extra_vars)
             end
      lambda_args = [env: self, vars: vars]
      vars.expand_varref(varref, lambda_args)
    end
def expand_varref(varref, extra_vars = nil)
      vars = if extra_vars.nil?
               @varset
             else
               @varset.merge(extra_vars)
             end
      lambda_args = [env: self, vars: vars]
      vars.expand_varref(varref, lambda_args)
    end
Expand a construction variable reference.

 @param varref [nil, String, Array, Proc, Symbol, TrueClass, FalseClass] Variable reference to expand.
 @param extra_vars [Hash, VarSet]
   Extra variables to use in addition to (or replace) the Environment's
   construction variables when expanding the variable reference.

 @return [nil, String, Array, Symbol, TrueClass, FalseClass] Expansion of the variable reference.