Synopsis:
   $mid(<offset> <length> <text>)

Technical:
   This function returns the input text, starting at the specified offset,
   and not exceeding the specified length.  If the length specified would
   extend beyond the end of the text, it is adjusted accordingly.  The
   offset is counted from 0 (zero).

Practical:
   This function takes an arbitrary input string, and returns a slice of
   it.  This might be used to strip the first few characters off of a
   string, regardless of what they might be.

Returns:
   slice of input text, or nothing if error

Examples:
   $mid(4 3 hello there)                returns "o t"
   $mid(7 100 this is a long string)    returns " a long string"

See Also:
   left(6); right(6)