Input in interactive mode is in many respects treated like a special S procedure: variables (including procedure-local variables) can be declared and created, a string of commands, separated by semicolons, can be entered as well as control structure blocks. For instance, if the WHILE command is entered at the terminal, no subsequent command will be executed immediately; they will be stored temporarily until the corresponding END-WHILE command is entered. The control structure can then be analyzed and executed. User support for interactive entry of control structures is provided by the fact that the system prompt always indicates which block needs to be closed next. This enables the interactive user to “try out” loops, condition constructs and variable access interactively prior to including them in a procedure.
There are, of course, a few restrictions that do not apply to “genuine” S procedures: for instance, GOTO cannot be used in interactive mode unless as part of block entries, continuation lines cannot start with insignificant blanks (since the slash need not be entered), and data lines in blocks can be created by means of SEND-DATA only (again because the slash, which serves to distinguish command lines from data lines, may be omitted).
The rules for interactive command input are described in more detail as of "SDF-P commands in interactive mode ".