title: How to create two dimensional set objects under pyomo.environ module
body: <p>I tried to create a LP model by using pyomo.environ. However, I'm having a hard time on creating sets. For my problem, I have to create two sets. One set is from a bunch of nodes, and the other one is from several arcs between nodes. I create a network by using Networkx to store my nodes and arcs.
  The node data is saved like (Longitude, Latitude) in tuple form. The arcs are saved as (nodeA, nodeB), where nodeA and nodeB are both coordinates in tuple. </p>
  
  <p>So, a node is something like:</p>
  
  <p>(-97.97516252657978, 30.342243012086083)</p>
  
  <p>And, an arc is something like: </p>
  
  <p>((-97.97516252657978, 30.342243012086083),
   (-97.976196300350608, 30.34247219922803))</p>
  
  <p>The way I tried to create a set is as following:</p>
  
  <pre><code># import pyomo.envrion as pe
  # create a model m 
  m = pe.ConcreteModel()
  # network is an object I created by Networkx module
  m.node_set = pe.Set(initialize= self.network.nodes()) 
  m.arc_set = pe.Set(initialize= self.network.edges())
  </code></pre>
  
  <p>However, I kept getting an error message on arc_set.</p>
  
  <pre><code>ValueError: The value=(-97.97516252657978, 30.342243012086083,
  -97.976196300350608, 30.34247219922803) does not have dimension=2, 
  which is needed for set=arc_set
  </code></pre>
  
  <p>I found it's weird that somehow my arc_set turned into one tuple instead of two. Then I tried to convert my nodes and arcs into string but still got the error.
  Could somebody show me some hint? Or how do delete this bug? 
  Thanks!</p>