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Groups a number of responses 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 responses: "a i", "front", 1, the new Confusion will be:
      front    i    ! The new response labels
    u     7    2    ! Responses on stimulus `u`
    i     5    4    ! Responses on stimulus `i`
    a     5    4    ! Responses on stimulus `a`
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