The test character x may be any one of l, n, m, or M, which indicate a "true" result if the corresponding escape sequence would return a non-empty value; or may be a, which indicates a "true" result if the watched user has logged in, or "false" if he has logged out. Other characters evaluate to neither true nor false; the entire expression is omitted in this case.
If the result is "true", then the true-text is formatted according to the rules above and printed, and the false-text is skipped. If "false", the true-text is skipped and the false-text is formatted and printed. Either or both of the branches may be empty, but both separators must be present in any case.