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850f327713
motivating example:
```nim
iterator p(a: openArray[char]): int =
if a.len != 0:
if a[0] != '/':
discard
for t in p(""): discard
```
The compiler wants to evaluate `a[0]` at compile time even though it is
protected by the if statement above it. Similarly expressions like
`a.len != 0 and a[0] == '/'` have problems. It seems like the logic in
semfold needs to be more aware of branches to positively identify when
it is okay to fail compilation in these scenarios. It's a bit tough
though because it may be the case that non-constant expressions in
branching logic can properly protect some constant expressions.