Problem "fixing" parameters

[GoogleCodeExporter]

What steps will reproduce the problem?

Define a standard Chi2 function for 3 parameters. Trying to fix one of the 
parameters and then run m.minos()

Example code :

import minuit

data =  [50,  12,   -8]
sigma = [13,     0.07,    0.1]

def chi2(a,b,c) :
    return   (data[0]-a)**2/sigma[0]**2 \
            +(data[1]-b)**2/sigma[1]**2 \
            +(data[2]-c)**2/sigma[2]**2

m = minuit.Minuit(chi2)
m.values["a"] = 50
m.fixed["a"] = True

m.migrad()
m.minos()

What is the expected output? What do you see instead?

Expect no errors, equivalent to just having fixed 'a' from the start. Instead 
get the following error:

python: MnMinos.cpp:101: MnCross MnMinos::loval(unsigned int, unsigned int) 
const: Assertion `!theMinimum.userState().parameter(par).isFixed()' failed.
Abort

What version of the product are you using? On what operating system?

Using pyminuit 1.1.2 with Minuit-1_7_9 on SLC6.4 (x64)

Please provide any additional information below.

I've been unable to fix parameters in combination with Minos or Contour for a 
variety of different functions/data

Original issue reported on code.google.com by rknegj...@gmail.com on 16 Sep 2010 at 3:22

[GoogleCodeExporter]

I have resolved this issue now. Calling minos() tries to find sigma=+/- 1 
errors for the fixed value as well and crashes in the proces. This can easily 
be avoided by calling minos("param",N) for each non-fixed parameter manually.

Great work with the minuit-python interface btw!

Original comment by rknegj...@gmail.com on 21 Sep 2010 at 12:16