Original Enhancement
Let a$ be a string expression，m, n be numeric expressions below.

Functions whose result is a string

SUBSTR$(a$,m,n) | Substring of a$ from m-th character to n-th character.
MID$(a$,m,n) | n characters of a$ from m-th character.
LEFT$(a$,n) | The first n characters of a$
RIGHT$(a$,n) | The last n characters of a$

The above functions are defined as follows.

    DEF SUBSTR$(a$,m,n)=a$(m:n)
    DEF MID$(a$,m,n)=a$(m:m+n-1)
    DEF LEFT$(a$,n)=a$(1:n)
    DEF RIGHT$(a$,n)=a$(LEN(a$)-n+1:LEN(a$))

Functions whose result is numeric.

BLEN(a$) | The length of a$ in bytes. This function is prepared for handling multi-byte characters.