Short: Rethink the 'mapping += mapping' semantics.
From: Lars
Date: 2002-05-07
Type: Feature
State: New

'mapping += mapping' should work like repeated calls to m_add(): the width of
the destination mapping is unchanged (unless generic empty mapping), the width
of the data added will be adjusted.

Possible way: + and += keep sizes and fail on size mismatch (with special
consideration of the empty mapping); | and |= merge even mappings is different
size.