docu$ -- special array, containing the contents of all docu-statement within the program

a$=docu$(1)

Before your program is executed, yab collects the content of all the doc-statements within your program within this 1-dimensional array (well only those within the main-program, libraries are skipped).
You may use the arraysize function to find out, how many lines it contains.

docu 
docu  This program reads two numbers 
docu  and adds them. 
docu 

rem retrieve and print the embedded documentation 
for a=1 to arraysize(docu$(),1) 
  print docu$(a) 
next a 

input "First number: " b 
input "Second number: " c 

print "The sum of ",b," and ",c," is ",b+c 

This program uses the embedded documentation to issue a usage-message.