6.20: adda/type"float/automated range tracking:
. the float type should be replaced with the real range;
ie, as users of scientific calculators,
we're supposed to be doing this in our heads,
but a friendly calculator needs to automate this .
. for literals there could be an implied range
given by +|- 1 of the most significant missing digit,
eg, 1 = 0.9..1.1 because
it's missing 0.x so it's 1 +|- 0.1 .
. after literals the range cannot be implied;
we must keep track of how much operations are widening it .