allow pipes to be duplicated

add suppressed status type and cause modifications to use it
	26 nov 1997


if file ends inside comment, can segfault on c_state::set_previous

clean up clock stuff
  clock inheritance
  make lists of secondary_clocks/targets more efficient in adds/removes

fix dc_constraint_link::rehash
	disallow_intersection followed by allow_intersection
	could cause problems when multiple links occur between same bodies
	and rehashing changes source or target or both. some intersections 
	allowed and then disallowed by subsequenent rehash of another link

fix create_attachment.  always attaches com to com 
	regardless of relative position/orientation

replace list_set with rehashable label_set in parse_finite_set for things like 
	set( a, b, c ).  label_set already defined but some member functions 
	need to be written


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28Oct97 - fix abbreviations in unit parser

( ( 0.01 + 0.04 ) ( kg/m2 ) )  causes error

make sure ==, !=, use coeff
