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1.13	The Stack.

	GEOMED commands  take their  arguments from  and leave  their
results in a push  down stack of bodies,  faces,  edges and vertices.
The contents of the stack are displayed on the left hand side  of the
display screen.  Every kind of  node has a default  print identifier,
for the first body created a `B1' is displayed in the stack. Making a
second cube will push a  `B2' into the stack. To retrieve `B1'  use the
"↑" stack pop command or the "α↑" stack rotate up  or "α↓" stack rotate down
or "↔" the swap top of stack command. The "↓" command pushs the stack
down and places in the new top the entity  that was previously top of stack.

1.14	The Face, Edge and Vertex Rings.

	A polyhedron consists  of three circular double  linked lists
(called rings); there is  a ring of faces, a ring of edges and a ring
of vertices. The head of each ring is the body node. The rings can be
traveled by  using the commands "<"  and ">" to go  forwards and back
through the face ring, the commands "≤" and  "≥" for the edge ring, and
the commands  "∨" and "∧"  for the  vertex ring.  Notice that when  a
face,  edge  or  vertex  node   is  in  the  top  of  the  stack  the
corresponding entity is intensified in the drawing. 

1.15	Hidden Line Elimination.

	Typing ALT-MODE causes your drawing  to be displayed with its
hidden lines eliminated;
as is illustrated by the two pictures of a log cabin in the woods.
To stay in hidden line display mode use the
"_" underbar command;  the "β_" returns the  display mode default  to
display all the lines. The hidden line eliminator, named OCCULT, does
not display concave faces correctly; and  for the
sake of  speed it  does not  check for them. The  faces of  a
polyhedra can be forced convex  by applying the "$" command to a body
or to a face.

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