Append the string body to the end of the string currently
being concatenated as the last string in the string buffer,
and update its count field. If there is no concatenating
string, start one. An error is thrown if the size of the
combined string would be larger than MAX_MCOUNT or if there
is not enough room in string space even after a garbage
If garbage collection occurs, a$ remains valid even when
it is in the string buffer.
When there is a concatenating string, concatenation is the
only basic string operation that can copy a string into the
string buffer. "cat"
NOTE: It is left to the user to define special concatenating