817bd51d

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Take the assertion scope from the keyword rather than overwriting it

The assertion production hardcoded AssertionScopeReal and extra_assertion then
rewrote it, so the first write was always dead and a third way to build an
assertion that forgot the rewriter would have silently got Real.  The
production now yields a function awaiting the scope, so there is one write and
nowhere to forget.

The assertion failure error also names which of the two balances it computed.
'Calculated:' was unambiguous only while there was one balance to mean.

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