This example shows how to construct a molecular hierarchy consisting of two proteins, and then apply connectivity restraint obtained from pulldown experimental data to these two proteins. 

On the following figure, we see an XML representation of the molecular hierarchy.

\include eg1_representation.xml

Each level of molecular hierarchy corresponds to an XML tag. Each tag can have
optional attributes. In this example, each protein has a unique id. Protein1 has
2 chains. The first chain is constructed from the PDB file using
IMP::atom::CAlphaSelector. The second chain is represented by a sphere fragment
that is big enough to contain 10 residues. Protein2 is constructed using the 
default selector, IMP::atom::NonWaterNonHydrogenSelector.

The following figure shows the definition of the restraint.

\include eg1_restraint.xml

The pulldown restraint is applied to Protein1 and Protein2.

The following Python script demonstrates the process of loading our data into \imp model.