A better DSL for flow queries

[nsbgn]

Right now, querying workflows is done using Flows, which are constructed by abusing Python's >> operator and ... ellipsis type. This is not future-proof, because it leaves very little room for adding functionality, and there are some other issues (such as: how do you express at what node a flow starts?)

A LINQ-style domain-specific language within Python would probably fit better.

[nsbgn]

A format like the following would probably be most:

Flow(a, b & Flow(..., c, d, ...))

Or perhaps 'Seq' instead of 'Flow'?

[nsbgn]

Perhaps best to just use lists for branches and tuples for sequences.

Flow(R3a(Obj, Reg, Ratio), ..., apply, [
        (..., ratio),
        (..., groupby, [(..., size), (..., pi1, R2(L, _))]),
        (..., apply, [(..., size), (..., R2(Obj, Reg))])
    ]
)

[nsbgn]

Closed 6bee39b

[nsbgn]

Note that the reflected << operator (__rrshift__) could also have been used to allow ... in the first position. The solution I went with is still preferable, though, because it's more concise and idiomatic (at the small cost of not having an intuitive visual indicator for the direction of flow).