Fexl has built-in functions to handle signed integers of arbitrary size, also known as "big numbers" or simply "bn" values.
The functions include:
The b20.fxl file runs some tests, including the computation of 2^256, a 78 digit number:
115792089237316195423570985008687907853269984665640564039457584007913129639936
The reference output is here.
Other tests of big numbers include b48.fxl and partition.fxl .