Return Less than of series and other, element-wise (binary operator lt).
Equivalent to series < other, but with support to substitute a fill_value for
missing data in one of the inputs.
series < other
Fill existing missing (NaN) values, and any new element needed for
successful Series alignment, with this value before computation.
If data in both corresponding Series locations is missing
the result will be missing.
Broadcast across a level, matching Index values on the
passed MultiIndex level.
The result of the operation.