p=2.5 k=2: 39 feasible random chains, max (Phi-G)/G = -1.094e-04; after local ascent: -3.004e-12
p=2.5 k=3: 2 feasible random chains, max (Phi-G)/G = -6.600e-03; after local ascent: 1.177e-12
--- stderr (rerun 2026-09-30):
Traceback (most recent call last):
  File "chains.py", line 37, in <module>
    b, s0 = wchain
    ^^^^^
TypeError: cannot unpack non-iterable NoneType object
