Add a delegating multifunction pattern

python/dune/perftool/ufl/delegate.py

+""" Implement a design pattern for a multifunction that delegates to another
+multifunction and gets back control if the delegate does not define a handler.
+This avoids writing isinstance-if-orgies in handlers
+"""
+
+
+def delegate(Delegate, *args, **kwargs):
+    assert(isinstance(Delegate, type))
+    assert(Delegate.expr == Delegate.undefined)
+
+    class MyDelegate(Delegate):
+        def __init__(self, *a, **ka):
+            Delegate.__init__(self, *a, **ka)
+
+        def expr(s, *a, **ka):
+            s._back(*a, **ka)
+
+    def _handler(s, *a, **ka):
+        delegate_instance = MyDelegate(*args, **kwargs)
+        delegate_instance._back = s
+        return delegate_instance(*a, **ka)
+
+    return _handler