In run.c::assign(), assigning to $0 from $F, a field,
where F >= 2, produces an incoherent cell.
The assignment occurs in two steps, first the string value
and then the float. When the string value is assigned to $0,
setsval invalidates the fields. If FS hasn't changed, after
getfval rebuilds the fields, NF = 1 and F >= 2, therefore $F
is definitely uninitialized. The result is a float val of
0.0, producing a boolean false in the pattern expression.
Coercing a string comparison gives the expected result
because the incoherent cell has the correct string
value, which is not empty and evaluates to true.
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