A lambda expression may optionally have a documentation string just after the lambda list. See Documentation , for how the documentation-string is accessed. It is a good idea to provide documentation strings for all the functions in your program, even those that are only called from within your program.
Documentation strings are like comments, except that they are easier to access.
The first line of the documentation string should stand on its own, because apropos displays just this first line. It should consist of one or two complete sentences that summarize the function's purpose. The start of the documentation string is usually indented in the source file, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up in the program source. This is a mistake.