substpat (should have been called "subst" for simplicity but that's already
used by the shell :-( ) can substitute a string for any occurence of a
pattern in a character stream. It reads from stdin and writes to stdout so
it can be used in pipes.
A typical invocation: substpat ^[Aa]ssign ";assign" <s:startup-sequence
This would comment out all lines starting with "assign" in your
startup-sequence and write the resulting file to stdout.
Substituting multiple patterns with different strings requires a pipe:
substpat ^[Aa]ssign ";assign" <s:startup-sequence | substpat "REMOVE ?" ""
(comment out assigns and kill all "REMOVE" keywords)

 PERMSSN    UID  GID    PACKED    SIZE  RATIO     CRC       STAMP          NAME
---------- ----------- ------- ------- ------ ---------- ------------ -------------
[generic]                  482     881  54.7% -lh5- 0712 Feb 26  1998 substpat.readme
[generic]                 1914    2816  68.0% -lh5- d754 Feb 26  1998 substpat/substpat
[generic]                 1789    4009  44.6% -lh5- e85c Feb 26  1998 substpat/substpat.c
[generic]                  150     250  60.0% -lh5- 5d1b Feb 26  1998 substpat/SCOPTIONS
---------- ----------- ------- ------- ------ ---------- ------------ -------------
 Total         4 files    4335    7956  54.5%            Feb 27  1998