array of maps that define the sequence of things to do. Each map has at least properties: "function" name of function supported by the tracking store "params" a map of additional parameters to that function. The output of each pipeline element is fed as input to the next stage. The initial dataset is empty, so typically the first function in the pipeline is a generator of data from the store.
Returns
non-null
value if there is a result, null if there is an error