(hdefine geometry|camera|window|appearance|image|transform|ntransform name value)(hdefine <type> <name> <value>)
is generally equivalent to
(read <type> { define <name> <value> })
except that the assignment is done when hdefine is executed, (possibly
not at all if inside a conditional statement), while the read
... define performs assignment as soon as the text is
read. See References. See (read ...). See (hdelete ...).