Four models, A, B, C, & D, pass messages between them, splitting and joining variables in the messages between them.  Model A receives input from a tab delimited table and sends two output variables which are split between models B & C according to the `output_pattern: 'scatter'` YAML parameter. The outputs from Models B & C are then merged and sent to model D according to the `input_pattern: 'gather'` YAML parameter. The output from model D is then sent to a table.
