function bnode = mk_input_node(bnet, self)
% MK_INPUT_NODE Make an observed root node without a prior distribution.
% bnode = mk_input_node(bnet, self)

if ~isempty(parents(bnet.adj_mat, self))
  error('input node cannot have any parents');
end
bnode.self = self; 
bnode.adjustable = 0;
bnode.type = 'input';
