----------------------------------------
|context> => |context: simple Feynman diagram>

supported-ops |e1> => |op: electron>
electron |e1> => |gamma 1>

supported-ops |gamma 1> => |op: electron> + |op: photon>
electron |gamma 1> => |e2>
photon |gamma 1> => |gamma 2>

supported-ops |e3> => |op: electron>
electron |e3> => |gamma 2>

supported-ops |gamma 2> => |op: electron>
electron |gamma 2> => |e4>
----------------------------------------