doc -- special comment, which might be retrieved by the program itself

 
doc   This is a comment 
docu  This is another comment 

Introduces a comment, which spans up to the end of the line. But other than the rem-comment, any docu-comment is collected within the special docu$-array and might be retrieved later on. Moreover you might invoke yab -docu foo.yab on the commandline to retrieve the embedded documentation within the program foo.yab.
Instead of doc you may just as well write docu or even documentation.

rem   Hi, this has been written by me 
rem 
doc   This program asks for a number and 
doc   prints this number multiplied with 2 
rem 
rem   Print out rhe above message 
for a=1 to arraysize(docu$()):print docu$(a):next a 

rem   Read and print the number 
input "Please input a number: " x 
print x*2 

This program uses the comments within its code to print out a help message for the user.
The contents of the doc-lines are retrieved from the docu$-array; if you do not want a comment to be collected within this array, use the rem-statement instead.