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Groups a number of stimuli into one new category.
Given the following selected Confusion:
          u    i    a   ! The response labels
    u     6    2    1   ! Responses on stimulus `u`
    i     3    4    2   ! Responses on stimulus `i`
    a     1    4    4   ! Responses on stimulus `a`
After the command Group stimuli: "u i", "high", 1, the new Confusion will be:
             u    i    a   ! The response labels
    high     9    6    3   ! Responses on stimulus group `high`
    a        1    4    4   ! Responses on stimulus `a`
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