     Whenever you make a mistake while trying to invoke a "command", an error
message is printed to tell you what went wrong.  If you misspell a command
name, a message like the following will be printed:

Segment such_and_so not found.

     If you make a mistake in the syntax of the "command line" or misspell the
name of an "argument," you will get a message that is preceded by the
name of the command you invoked.  For example, if you use the -directory
"control argument" with the print command, the following error message will be
printed:  

print:  Specified control argument is not accepted. -directory

     There are other types of error messages you may get, and they don't all
result from mistakes you make.  When you are unsure what a message means, the
best advice is to ask someone who does know.
