function structure =  SS_eqn(name,Bond_number,Bonds,Direction,cr,args, ...
			     structure,filenum);

  disp("SS_eqn");
  name,Bond_number,Bonds,Direction

  ## Set up globals to count the component inputs and outputs. This relies on
  ## the named SS (the ports) being in the correct order. Using globals here
  ## avoids changing the common argument list for all _eqn files for something
  ## which is only used for named SS components.
  global local_u_index
  global local_y_index
  global at_top_level

  ## SS_eqn - equations for SS component
  ## 
  ##     ####################################### 
  ##     ###### Model Transformation Tools #####
  ##     ######################################
  ## 
  ## Matlab function  SS_eqn
  ## structure =  SS_eqn(name,bond_number,bonds,direction,cr,args, ...
  ##    structure,filenum);


  ## Copyright (c) P.J. Gawthrop, 1996.

  
  ## ###############################################################
  ## ### Version control history
  ## ###############################################################
  ## ### $Id$
  ## ### $Log$
  ## ### Revision 1.27  2005/03/21 10:34:47  gawthrop
  ## ### Tidied.
  ## ###
  ## ### Revision 1.26  2004/08/29 13:11:21  geraint
  ## ### Writes MTTyz vector in the same format as MTTx and MTTy: MTTyz($d,1)
  ## ##
  ## ### Revision 1.25  2003/01/07 18:09:21  geraint
  ## ### More informative error message.
  ## ##
  ## ### Revision 1.24  1999/10/22 02:20:32  peterg
  ## ### Changed error handleing of number of attributes >2
  ## ##
  ## ### Revision 1.23  1998/12/03 16:46:16  peterg
  ## ### Deblanked attributes so that zero attribute works.
  ## ##
  ## ### Revision 1.22  1998/07/28 19:05:46  peterg
  ## ### Fixed a few bugs.
  ## ##
  ## ### Revision 1.21  1998/07/28 14:21:31  peterg
  ## ### Vector SS ports.
  ## ##
  ## ### Revision 1.20  1998/07/08 14:42:52  peterg
  ## ### Removed the annoying info message - it causes problems with big
  ## ### systems
  ## ##
  ## ### Revision 1.19  1998/07/08 11:33:54  peterg
  ## ### Replace mtt_info by mtt_error when appropriate
  ## ##
  ## ### Revision 1.18  1998/07/08 11:30:45  peterg
  ## ### Removed second (fileID) argument from mtt_info
  ## ##
  ## ### Revision 1.17  1998/07/04 07:15:44  peterg
  ## ### Back under RCS
  ## ##
  ## ### Revision 1.16  1998/04/11 19:07:16  peterg
  ## ### Now do named ports as ordinary ports iff at top level.
  ## ### --- not yet complete, need to pass necesssary info though to this
  ## ###     function
  ## ##
  ## ### Revision 1.15  1997/12/16 19:16:07  peterg
  ## ### Added unknown input to the effort part.
  ## ##
  ## ### Revision 1.14  1997/12/16 18:25:19  peterg
  ## ### Added unknown_input attribure to flow -- effort still needs doing
  ## ##
  ## ### Revision 1.13  1997/09/18 13:15:15  peterg
  ## ### Fixed incorrect error message flagging inappropriate flow outputs
  ## ### -- used to give the effort rather than the flow in the error message.
  ## ##
  ## ### Revision 1.12  1997/08/26 07:51:30  peterg
  ## ### Now counts the local input and outputs by order of appearence rather
  ## ### than by port number - it therfore handles ports with bicausality correctely.
  ## ##
  ## ### Revision 1.11  1997/05/09 08:21:07  peterg
  ## ### Explicit computation of port number -- avoids str2num
  ## ##
  ## ### Revision 1.10  1997/03/22  17:13:03  peterg
  ## ### Fixed bug for port nos. > 1 digit!
  ## ##
  ## ### Revision 1.9  1997/03/22  15:50:59  peterg
  ## ### Changed #1.0f to %d format.
  ## ##
  ## ### Revision 1.8  1996/12/10 16:52:29  peterg
  ## ### Detect null string using strcmp, not length.
  ## ### Put filnum argument to mtt_info.
  ## ##
  ## ### Revision 1.7  1996/12/07 17:17:40  peterg
  ## ### Added some ;
  ## ##
  ## ### Revision 1.6  1996/12/05 09:49:09  peterg
  ## ### Explicit computation of port number from string.
  ## ##
  ## ### Revision 1.5  1996/12/04 21:27:53  peterg
  ## ### Replaced str2num by sprintf
  ## ##
  ## ### Revision 1.4  1996/08/18  20:06:21  peter
  ## ### Included zero outputs.
  ## ##
  ## ### Revision 1.3  1996/08/09 14:08:04  peter
  ## ### Empty effort and flow attributes replaced by 'external'.
  ## ##
  ## ### Revision 1.2  1996/08/08 16:38:19  peter
  ## ### Changed to filenumer type of fprintf
  ## ##
  ## ### Revision 1.1  1996/08/08 16:19:08  peter
  ## ### Initial revision
  ## ##
  ## ###############################################################


  ## The string 'cr' contains the effort attributes;  
  ## 'arg' contains the flow attributes. 
  ## external indicates an external variable
  ## internal indicates an internal variable
  ## anything else is the name of a constant.
  ## Blank is taken to be external
  ## If its a numbered port:
  ##     cr contains 'MTT_port'
  ##     arg contains port number

  if (strcmp(cr,"SS"))		# Then its the standard file
    a = char(strsplit(args,","))
    [N,M]=size(a);
    if (N~=2)			# Must have 2 arguments
      mtt_error(sprintf("SS should have 2 args not %i", N));
    endif
    effort_attribute = deblank(a(1,:));
    flow_attribute   = deblank(a(2,:));
  else				# Old style file
    effort_attribute = cr;
    flow_attribute = args;
  endif

  ## Default attributes
  if strcmp(effort_attribute,"")
    effort_attribute = "external";
  endif

  if strcmp(flow_attribute,"")
    flow_attribute = "external";
  endif

  inputs = structure(3);
  outputs = structure(4);
  zero_outputs = structure(5);
  unknown_inputs = structure(6);
  
  n_bonds = length(Bond_number)	# Multi port?
  for i=1:n_bonds		# Loop over all the bonds
    bond_number = Bond_number(i);
    bonds = Bonds(i,:);
    direction = Direction(i,:);
    Named_Port = name(1)=="[";	
    if Named_Port
      sname = name(2:length(name)); #Strip the []
    else
      sname = name;
    endif
    
    if Named_Port&&~at_top_level # It's a named port
      ## Effort 
      if bonds(1,1)==-1		# Source
	local_u_index = local_u_index + 1
	fprintf(filenum, "%s := %s_MTTu%d;\n", ...
		varname(sname, bond_number,1), sname, local_u_index);
      else # Sensor
	local_y_index = local_y_index + 1
	fprintf(filenum, "%s_MTTy%d := %s;\n", ...
		sname, local_y_index, varname(sname, bond_number,1));
      endif
      
      ## Flow 
      if bonds(1,2)==1		# Source
	local_u_index = local_u_index + 1
	fprintf(filenum, "%s := %s_MTTu%d;\n", ...
		varname(sname, bond_number,-1), sname, local_u_index);
      else # Sensor
	local_y_index = local_y_index + 1
	fprintf(filenum, "%s_MTTy%d := %s;\n", ...
		sname, local_y_index, varname(sname, bond_number,-1));
      endif
    else			# Now do SS which are not ports
      ## Effort
      if strcmp(effort_attribute, "external")
	if bonds(1,1)==-1	# Source
	  inputs = inputs+1;
	  fprintf(filenum, "%s := MTTu(%d,1);\n", ...
		  varname(sname, bond_number,1),inputs);
	else			# Sensor
	  outputs = outputs+1;
	  fprintf(filenum, "MTTy(%d,1) := %s;\n", ...
		  outputs, varname(sname, bond_number,1));
	endif
      elseif strcmp(effort_attribute, "unknown") # Unknown input
	unknown_inputs = unknown_inputs + 1;
	fprintf(filenum, "%s := MTTUi%d;\n", ...
		varname(sname, bond_number,1), unknown_inputs);
      elseif strcmp(effort_attribute, "internal")
	## Do nothing
      else 
	if bonds(1,1)==-1	# Named or unknown source
	  fprintf(filenum, "%s := %s;\n", ...
		  varname(sname, bond_number,1), effort_attribute);
	else # Sensor
	  if strcmp(effort_attribute, "zero") #Zero output
	    zero_outputs = zero_outputs + 1;
	    fprintf(filenum, "MTTyz(%d,1) := %s;\n", ...
		    zero_outputs, varname(sname, bond_number,1));
	  else
	    mtt_error([effort_attribute, " not appropriate for an output (", name, ")"]);
	  endif
	endif
      endif
      
      ## Flow
      if strcmp(flow_attribute, "external")
	if bonds(1,2)==1	# Source
	  inputs = inputs+1
	  fprintf(filenum, "%s := MTTu(%d,1);\n", varname(sname, bond_number,-1),inputs);
	else			# Sensor
	  outputs = outputs+1
	  fprintf(filenum, "MTTy(%d,1) := %s;\n", outputs, ...
		  varname(sname, bond_number,-1));
	endif
      elseif strcmp(flow_attribute, "unknown") # Unknown input
	unknown_inputs = unknown_inputs + 1
	fprintf(filenum, "%s := MTTUi%d;\n", ...
		varname(sname, bond_number,-1), unknown_inputs);
      elseif strcmp(flow_attribute, "internal")
	## Do nothing
      else			# Named constant
	if bonds(1,2)==1	# Source
	  fprintf(filenum, "%s := %s;\n", ...
		  varname(sname, bond_number,-1), flow_attribute);
	else			# Sensor
	  if strcmp(flow_attribute, "zero") #Zero output
	    zero_outputs = zero_outputs + 1
	    fprintf(filenum, "MTTyz(%d,1) := %s;\n", ...
		    zero_outputs, varname(sname, bond_number,-1));
	  else
	    mtt_error([flow_attribute, " not appropriate for an output (", name, ")"]);
	  endif
	endif
      endif
    endif
    
    structure(3) = inputs;
    structure(4) = outputs;
    structure(5) = zero_outputs;
    structure(6) = unknown_inputs;
    
  endfor
endfunction