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Re: Namestring&pathstring returning shared structure

 From as early as 1972, Interlisp in all implementations performed "copy
on write for substrings of pnames". This required neither BIBOP or page
tables or tag bits. 

This is one "is an error" condition that is fairly painful: it isn't
determinable by even the most careful amount of flow analysis, much less
a simple syntactic check.

To change "it is an error to destructively modify the pname of a
symbol", it is necessary to either introduce copy-on-write or else
change it to "signals an error". If the former is controversial, would
you accept the latter?