Copy the external string value whose body address and count
are on the parameter stack into the string buffer and push it
onto the string stack. Errors are thrown if the count is
larger than MAX_MCOUNT, if there is not enough room in string
space, even after garbage collection, or if there is an
unterminated string concatenation. The input external string
need not exist as a measured string.
NOTE: MAX_MCOUNT is the largest size the count field of a
measured string can hold, e.g., 255, 64K-1, or 4,096M-1. It
is returned by: S" /DYNAMIC-STRING" ENVIRONMENT?
WARNING: This word should not be used when the input string
is a bound string because the copy operation may generate a
garbage collection which invalidates its MSA.