Kind inference doesn't work when right-hand sides exclude function calls

Kind inference fails for the following:

def bw_euler():
    # Set up variables
    y = var("<state>y")
    t = var("<t>")
    dt = var("<dt>")

    with CodeBuilder("main") as cb:
        cb(y, y * 1/(1 + 10 * dt))
        cb(t, t + dt)
        cb.yield_state(y, "y", t, "t")

    return DAGCode.from_phases_list([cb.as_execution_phase("main")], "main")

This code could be a legitimate method. It's the backward Euler method for the right-hand side y' = -10 y.

The error is

Left-over statements in kind inference:
[main] <state>y <- <state>y / (1 + 10*<dt>)
  -> nothing known about '<state>y'
Traceback (most recent call last):
  File "bw_euler.py", line 54, in <module>
    main()
  File "bw_euler.py", line 50, in main
    infer_kinds(bw_euler())
  File "/Users/matt/src/dagrt/dagrt/data.py", line 634, in infer_kinds
    return kind_finder(names, phases)
  File "/Users/matt/src/dagrt/dagrt/data.py", line 529, in __call__
    raise RuntimeError("failed to infer kinds")
RuntimeError: failed to infer kinds

(infer_kinds is a new function I'm adding as part of #34 (closed).)

It's not clear to me how to repair this. Maybe this demonstrates that not all programs have an unambiguous type, so hints are sometimes needed?