EWD249 - 68

CLEARFIRST
begin
build: {clear; setmarks};
instr clear(image), setmarks(image)
end

where the action "clear" assigns to image the value corresponding to a picture of fifty blank lines, where the action "setmarks" adjusts the initial value of image to the one in which the thousand (or less) marks of the curve have been added.

Again, CLEARFIRST is a machine for which alternative programs could have been written, e.g.

build: {clear}

would make sense, but would produce fifty blank lines as output;

build: {setmarks; clear}

would contain an undefined operation;

build: {clear; clear; setmarks}

would contain a superfluous operation, just as

build: {clear; setmarks; setmarks}

would, because the second action "setmarks" would only add marks to the picture that would already be there and therefore would not change the value of "image".

(Note on notation used. The algorithm explaining "build" in terms of "clear" and "setmarks" does so without explicitly mentioning "image", because we do not wish to use the actual parameter notation in algorithms unless its actual combinatorial freedom is in fact used in this machine.

Furthermore, "build" being a one-parameter operation no separate identifier for its formal parameter has been introduced. Also this abbreviation on my part could turn out to be very unwise.)

The next step in the design of the computation -because it can be made without any further commitments- is to describe how the thousand marks of the curve will be dealt with in turn. For the time being I propose the following write-up: