Dylan Design Notes

#14: Union Allows Duplicates	(Clarification)

Version 1, March 1993
Copyright (c) 1993-1994, Apple Computer

This design note clarifies that the result of union is only 
guaranteed free of duplicates if no element appears more than once 
in a single argument sequence.  If the same element appears more 
than once in a single argument sequence, it may appear more than 
once in the result sequence.  This allows union to be implemented 
more efficiently.

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Replace the first sentence of the description of union on page 107 
of the manual with the following:

union returns a sequence containing every element of sequence1 and 
sequence2.  If the same element appears in both argument sequences, 
this will not cause it to appear twice in the result sequence.  
However, if the same element appears more than once in a single 
argument sequence, it may appear more than once in the result 
sequence.

