% Length constants
bp cc cm dd in mm pc pt

% Pair constants
down left origin right up

% Numeric constants
eps epsilon infinity _ % _ is -1, as per Knuth: "internal constant to make macros unreadable but shorter"

% Path constants
fullcircle halfcircle quartercircle unitsquare

% Pen constant
nullpen penrazor penspeck pensquare penstroke

% Picture constants
blankpicture nullpicture unitpixel

% String constant
ditto

% Transform constant
identity

% Other constants
proof relax smoke \ \\ ???

----------------------------------------------------

[
	["comment", "% Length constants"],

	["constant", "bp"],
	["constant", "cc"],
	["constant", "cm"],
	["constant", "dd"],
	["constant", "in"],
	["constant", "mm"],
	["constant", "pc"],
	["constant", "pt"],

	["comment", "% Pair constants"],

	["constant", "down"],
	["constant", "left"],
	["constant", "origin"],
	["constant", "right"],
	["constant", "up"],

	["comment", "% Numeric constants"],

	["constant", "eps"],
	["constant", "epsilon"],
	["constant", "infinity"],
	["constant", "_"],
	["comment", "% _ is -1, as per Knuth: \"internal constant to make macros unreadable but shorter\""],

	["comment", "% Path constants"],

	["constant", "fullcircle"],
	["constant", "halfcircle"],
	["constant", "quartercircle"],
	["constant", "unitsquare"],

	["comment", "% Pen constant"],

	["constant", "nullpen"],
	["constant", "penrazor"],
	["constant", "penspeck"],
	["constant", "pensquare"],
	["constant", "penstroke"],

	["comment", "% Picture constants"],

	["constant", "blankpicture"],
	["constant", "nullpicture"],
	["constant", "unitpixel"],

	["comment", "% String constant"],
	["constant", "ditto"],

	["comment", "% Transform constant"],
	["constant", "identity"],

	["comment", "% Other constants"],

	["constant", "proof"],
	["constant", "relax"],
	["constant", "smoke"],
	["constant", "\\"],
	["constant", "\\\\"],
	["constant", "???"]
]

----------------------------------------------------

Checks for constants best not to be redefined.