AT&T 6300 Programmer's Manual page 660

Converting Programs
SUBSTRINGS
In GWBASIC the following functions are used
to take substrings of strings:
LEFTS
MIDS
RIGHTS
Other forms, such as:
AS(I) (to access the Ith character in A$) or
AS(I,J) (to take a substring of A$ from
position I to
J)
should be changed as follows:
Other BASICs
XS=AS(I)
XS=AS(I,J)
GWBASIC
= XS=MIDS(AS,I,1)
= XS=MIDS(AS,I,J-I+1)
If the substring reference is on the left side of
an assignment and X$ is used to replace
characters in A$, then the conversion should
be carried out as follows:
C-4
Other BASICs
AS(I)=XS
AS(I,J)=XS
GWBASIC
= MIDS(AS,I,1)=XS
= MIDS(AS,I,J-I+1)=XS