function g=mat_2_graph(a,directed,mat)
// Copyright INRIA
[lhs,rhs]=argn(0)
if rhs==2 then 
  mat='node-arc'
else
  if rhs<>3 then error(39), end
end
// directed
if directed<>1&directed<>0 then
  error('Second argument must be 0 or 1')
end
// a
if type(a)<>5 then
  error('Matrix must be sparse')
end

[ij,v,mn]=spget(a);
n=mn(1)
m=mn(2)

// node-arc matrix
if mat=='node-arc' then
 if prod(size(v))<>2*m then
   error('Matrix has wrong number of elements')
 end
 if directed==1 then
 
   if norm(ones(1,n)*a)<>0 then
     error('Matrix is not a node-arc matrix')
   end
 
   itab=find(v==1)
   jtab=find(v==-1)
 
   if prod(size(itab))<>prod(size(jtab)) then
     error('Matrix is not a node-arc matrix')  
   end
 
   he=ij(itab,:); ta=ij(jtab,:)
   [s,k]=sort(he(:,2)); he=he(k,:); he=he(:,1)
   ma=prod(size(k)); he=he(ma:-1:1)
   [s,k]=sort(ta(:,2)); ta=ta(k,:); ta=ta(:,1)
   ma=prod(size(k)); ta=ta(ma:-1:1)
   g=make_graph('foo',1,n,he',ta')
 
 else
   
   oa=ones(1,n)*a
   if max(oa)>2|min(oa)<2 then
     error('Matrix is not a node-arc matrix')    
   end
   
   [ijp,vp,mnp]=spget(a');
   s=size(ijp)
   he=ijp(2:2:s(1),2)
   ta=ijp(1:2:s(1),2)
   g=make_graph('foo',0,n,he',ta')
   
 end
elseif mat=='node-node'
// node-node matrix
  if m<>n then error('Matrix must be a square matrix'), end 
  if min(v)<>1|max(v)<>1 then
    error('Matrix is not a node-node matrix')
  end
  if directed==1 then
    he=ij(:,1); ta=ij(:,2);
    g=make_graph('foo',1,n,he',ta')
  else
    ij=spget(tril(a)');
    he=ij(:,1); ta=ij(:,2);
    g=make_graph('foo',0,n,he',ta')
  end
else
 error('Third argument must be ""node-node"" or ""node-arc""')
end
