Is it really the case that each variable of FCN has to be a single character?

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What steps will reproduce the problem?
1. set up FCN to be a function of e.g. xa and xb
2. try migrad() on it
3. find a KeyError

e.g.
>>> import minuit
>>> def f( xa, xb ):
...     return ( xa**2 + xb**2 )
... 
>>> m = minuit.Minuit( f )
>>> m.values
{'xb': 0.0, 'xa': 0.0}
>>> m.migrad()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Parameter "x" is missing from values.'


What is the expected output? What do you see instead?
expected: normal running (above example, that xa = 0.0, xb = 0.0). instead, 
KeyError. changing xa to a and xb to b brings back expected behavior.

What version of the product are you using? On what operating system?
PyMinuit 1.2.1, SEAL-MINUIT 1.7.9, Python 3.3.0, Mac OSX 10.6

Please provide any additional information below.
I can work around it, but really? I cannot use x1, x1, x3, ... ?

Original issue reported on code.google.com by [email protected] on 4 Jan 2013 at 6:28