The next comment explains the division into four classes of arithmetic, relative to underflow. Value add_sub_tiny is nonzero when the arithmetic supports tiny, usually unnormalized, values. This is the case in arithmetic compliant with IEEE 754. Value underflow_threshold is set thus far by the call to tiny_values_and_difference(). It is zero if the arithmetic flushes underflowed values to zero, so Paranoia will set it as needed. It is nonzero when the arithmetic has a well-defined, usually normalized, value below which the arithmetic degrades to nonzero values, with some loss of precision.
[TODO: give examples of all four.]
# The Basic code switches on four cases depending on whether two values are
# zero or nonzero.
# add_sub_tiny: the difference between two tiny unequal values may
# underflow to zero
# underflow_threshold: intended to be the smallest normal number
# unaffected by underflow, but will not have been set yet for
# arithmetic flushes underflow abruptly to zero
#
# -----------CASES---- 4 3 2 1
# add_sub_tiny non0 non0 0 0
# underflow_threshold non0 0 non0 0
#
# In high-quality arithmetic, both values are nonzero, so it's just a matter
# of checking for gradual underflow and its quality.
# If on the other hand, either value is zero, a specific action is taken and
# then all three cases fall into an explicit check for differences of zero
# arising from unequal values.