.FUNCTION....: Substr (Substring) Function
.CATEGORY....: functions
.DISCUSSION..:
This function returns a substring of the string supplied in the /Q parm, starting in the column supplied in the /A parm, for a length supplied in the /B parm.

The start column specified in the /A parm may be a negative number; if it is, then the start column is calculated from the end of the string rather than from the beginning. This feature makes it easy to extract the rightmost characters of a string.

Specifically, when the ParmA is negative, the start position is calculated using the following formula:

• StartColumn = length(ParmQ) + 1 + ParmA

``` EXAMPLE....:  Fdate /Fsubstr  /A-2 /Qabcd
CALCULATION:  StartColumn = length(ParmQ) + 1 + ParmA
StartColumn =      4        + 1 + (-2)     = 3
```
• If /Q is omitted, Fdate reports an error.

• If /A is zero, Fdate reports an error.

• If /A is omitted, it defaults to 1 (that is, the start of the string).

• If /A is negative, and longer than the length of the string, then the start column is 1 (the start of the string)

• If /B (length) is so great that it exceed the length of the /Q input string, then only the remainder of the string is returned.

• If /B is zero, Fdate returns a null string.

• If /B is omitted, it defaults to 999.
.EXAMPLES....:
```          Fdate /Fsubstr /A7 /B2 /Q"STEVE FERG"   ===> "FE"
Fdate /Fsubstr /A7     /Q"STEVE FERG"   ===> "FERG"
Fdate /Fsubstr     /B5 /Q"STEVE FERG"   ===> "STEVE"
```
.EXAMPLES....: using a negative start position
```          Fdate /Fsubstr /a-6 /B3 /Q"1994 Jun 03"   ===> "Jun

rem extract the rightmost 6 characters of a string
Fdate /Fsubstr /a-6     /Q"1994 Jun 03"   ===> "Jun 03"

rem pad YEAR environment variable to the left with zeroes,
rem to make sure it is 4 bytes long
Fdate /Fsubstr /A-4 /q0000%year% /vyear
```
.DISCUSSION..:
REMEMBER that Fdate eliminates redundant spaces when obtaining its parameter input:

```           Fdate /Fsubstr /A7     /Q"STEVE FERG"      ===> "FERG"
Fdate /Fsubstr /A7     /Q"STEVE  FERG"     ===> "FERG"
Fdate /Fsubstr /A7     /Q"STEVE    FERG"   ===> "FERG"
```