1 10/17/84  reverse_index, rvindex
 2 
 3 Syntax as a command:  rvindex STRA STRB
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 5 
 6 Syntax as an active function:  [rvindex STRA STRB]
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 8 
 9 Function:  performs the same function as the index command/active
10 function, but in reverse order.
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12 
13 Notes:  The active function returns the index (character position) in
14 STRA of the beginning of the last occurrence of STRB (index uses the
15 first occurrence).  If STRB does not appear in STRA, 0 is returned.
16    [reverse_index STRA STRB]
17 is the same as--
18    [length STRA] - [index [reverse STRA] [reverse STRB]] + 2
19       - [length STRB]
20 when STRB appears in STRA.  It is 0 when STRB does not appear in STRA.