If unforgeable, might help solve Jessie this-capture issue.

See the section "Avoid this-capture Hazards" at https://github.com/Agoric/Jessie#avoid-this-capture-hazards

If we had an unforgeable test that a function cannot access its `this` argument, then we could consider other approaches to this this-capture hazard. For example, if we had a membrane between SES and Jessie, it could filter out this-sensitive SES functions from reaching Jessie code.

Attn @michaelfig