This feature is ~R directive which knows how to say really big numbers in English words.

Here is the example (I use SBCL + SLIME):

CL-USER> (format nil "~R" 1604872898756737459538)
"one sextillion six hundred four quintillion eight hundred seventy-two
quadrillion eight hundred ninety-eight trillion seven hundred fifty-six
billion seven hundred thirty-seven million four hundred fifty-nine
thousand five hundred thirty-eight"