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Returns the number of responses for the chosen stimulus (the sum of all the numbers in the row with this stimulus label).
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`
The command Get stimulus sum: "a" will return the number 9.
© djmw 20140117