Append an 'expect' command to the script. The set value of this
object should be the exact characters expected to be received on
the port opened with the 'open' command. The value could also be
a single wildcard character '*' which means any received content
will be accepted. The 'expect ' characters will be automatically
appended. Also a 'null terminator'(\0) will be automatically
appended. The null terminator and the 'expect ' characters will
be counted as characters in the script.