yapp stands for "yet another program printer", or "yet another
printing program", or maybe "yet another printer printer". yapp is a
program that prints out programs that print out.

That is, yapp takes whatever is piped into its standard input, and
produces on its standard output the source code for an INTERCAL
program that will, when compiled, produce the original file. The
created program will be self-contained; it will not need the system
library to compile. The reason for this is to maximize the amount of
data that yapp can encode into an INTERCAL program. The current
compiler has an upper bound on the number of commands one program can
have; thus yapp will not produce a compilable program if given more
than 3880 bytes.

Though yapp was intended to create standalone programs, it can also be
used to generate subroutines. (In fact, the first working version of
this program was used to generate the lines for the final draft.)  In
such a case, it is convenient to specify what character the data will
"follow". The easiest way to accomplish this is to replace the "DO .9
<- #0" in line 22 with "DO .9 <- <num>", where <num> is the
bit-reversed code of the character preceeding the data (#80 in the
case of a newline, for example). A more versatile modification is to
change line 22 to "DO WRITE IN .9", and then insert the character code
at the top of the file that is to be fed to yapp. Thus, if the data in
the file would follow a newline, the line "EIGHT OH" would be added to
the top of the data file.