The Pager

  Since the display of even a relatively simple indexed object can occupy several screens it became necessary to introduce a pager to handle the scrolling of output. By redefining a REDUCE primitive REDTEN is able to page the output, allowing the user a chance to see and examine each page at leisure without needing to be quick with the no-scroll key; and to abort the output if it is too long-winded, without needing to use the sometimes-dangerous abort sequence.

If the switch dopaging is on, any output of an algebraic nature will pause after a pre-determined number of lines. This number is stored in lisp:screenlines!*[22] and can be set by the user directly, from a startup file (assuming this is supported) or from the environment variable SCREENLINES (again, assuming this is supported). At each pause the prompt Continue? appears, asking the user for input before continuing. Valid inputs are (case is ignored):

• q or n --- A throw to the top-level input loop is made, thus aborting the output
• c --- continue the output with no more pauses
• d --- make the next page half-height
• other --- any other response continues the output, pausing after the next page.

For the pager to operate properly the up-cursor escape sequence upcursor!* must be correctly defined. This allows the pager code to overwrite the prompt and produce a seamless output.