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This patch fixes the problem by joining the metavar terms with '|'. So the help for the test case (adapted from an existing tuple test) looks like:
 usage: PROG [-h] W1 [W2 ...] [X1 [X2 ...]] Y1 Y2 Y3 [Z1]
 positional arguments:
 W1|W2 w
 X1|X2 x
 Y1|Y2|Y3 y
 Z1 z
Alternatives include:
- use ',',' ',or '/' instead of '|'
- use just the 1st item of the tuple
- use the default (non-tuple) metavar in the help line
These all pass existing tests.
The last alternative would use:
 #metavar = '|'.join(metavar)
 if len(metavar)>1:
 metavar = default
 else:
 metavar = metavar[0]