potential term for restraining the distance between atoms and a plane.
Please use the helper function create_PlaneDistPot in the module 
<m planeDistTools> to simplify creation of this term.

constructor: PlaneDistPot(instanceName,
	                  oAtom, xAtom, yAtom,
                          restraintList,
		          sim                )

  instanceName is a user-specified identifier
  oAtom, xAtom, yAtom are atoms defining the plane.
  restraintList is an optional XPLOR-style restraint table (string).
  sim, is an optional <m simulation>.Simulation specification
  [they default to the current Simulation.]

methods
  planeNorm()                  - the current value of the plane normal.
  addRestraints(restraintList) - add the specified XPLOR-style restraints

  calcEnergy()                 - calc energy, returns the energy value.
  calcEnergyAndDerivs(derivs)  - calc energy, derivs, returns the energy value.

  rms()            - return the restraint rms violation
  numRestraints()  - return the numbder of restraints defined for this term.
  violations()     - return number of violations
  
  info()                    - current info about the state of this instance
  showRestraints(violated)  - return info on restraints. Argument violated is
                              boolean specifying whether to return only 
 			      violated restraints.
  showViolations()          - return string containing pretty-printed info
                              on all violated restraints.

 The following parameters can be set [defaults in square brackets]

  oAtom      - the plane-defining atoms
  xAtom  
  yAtom      
  nMono      - number of monomers [1]
  verbose    - if true, sporadically spit out info [False]
  scale      - scale factor [1]
  aveExp     - exponential to use in sum averaging [6]
  dOffset    - potential offset [0]
  hardExp    - exponential in hard region of potential [2]
  rSwitch    - distance at which to switch from hard form [0.5]
  softExp    - exponential in soft region of potential [1]
  asympSlope - slope in linear asymptotic region of pot [1]
  threshold  - threshold in violation calculation [0.5]
  potType    - type of potential: "soft" or "hard" [hard]
  aveType    - type of restraint averaging: "average", "center" or "sum" [sum]
  showAllRestraints - boolean which changes the behavior of showViolations. If 
                      this parameter is set to True, the behavior of
		      showViolations is modified such that all restraints are
      	              printed. Violated restraints are indicated by an
                      asterisk in the first column. [False]

 the above quantities may be retrieved using the member function form
 quantity(), while they are set using the form setQuantity(value).


 The energy function is defined as

	if pot=HARD or  r<rSwitch: 
		scale * r^hardExp
	pot=SOFT and r>rSwitch: 
		scale * (a + b / r^softExp + asympSlope * r)

	where
   	b =  rSwitch^(softExp+1) * 
   	    (asympSlope - hardExp * rSwitch^(hardExp-1)) / softExp
   	a = rSwitch^hardExp - b * rSwitch^(-softExp) - 
   	    asympSlope * rSwitch;
	
 The distance r is related to the calcualted restraint distance d by:
	if d < cdist - cMinus:      r = cdist - cMinus - d
	if d > cdist+cPlus-dOffset: r = cdist - ( d + cPlus - dOffset )
        otherwise, r = 0

 The restraint distance d is calculated as
   for aveType=average:
	d = [ sum_i | dot(q_i-p1, n) |^(-aveExp) / Ni ] ^(-1/aveExp)
     where q_i is the position of the i-th atom, and Ni is the number
of atoms in the sum. p1 is the position of oAtom, while n is the unit
vector in the direction 
     (p2-p1) X (p3-p1),
where p2 is xAtom.pos() and p3 is yAtom.pos()

   for aveType=sum:
	d = [ sum_i | dot(q_i-p1, n) |^(-aveExp) / nMono ] ^(-1/aveExp)
     where q_i is the position of the i-th atom. 

   for aveType=center:
	d = | dot(sum_i q_i/Ni - p1, n) |
      
XPLOR NOE restraint table format:

  ASSIgn <selection>  <cDist> <cMinus> <cPlus> ! optional comment

the statement can be split across muliple lines, but the comment, if
specified, should be on the same line at cPlus.