William Steele reported and solved this.

This test was provided by Lewis Muir, it fails with seq-2.1.5.

This is now configurable in test/validate/Makefile.

This tests that for, while, if, and if/else statements
can be nested to at least depth 40.

Thanks to Mark Rivers who suggested this test.

As turned out, the 2.0.x branch supported this, so it is added
for compatibility.

Implementing this meant that the invariants governing the
three-state values (none, single, multi) of the assign,
monitor, sync, and syncq members had to be redesigned.
More comments in analysis.c indicate where these laws are
established and where we depend on them (assert them).
Also added / changed corresponding tests in test/compiler.

A test that checks that such a warning is issued
is already in test/compiler.

Among other things this ensures that each call to malloc
results in a different pointer, which is necessary to
check for certain subtle bugs.