The Hardest Program I've Ever Written (2015)
Pretty printers are hard.    "We sat down one morning," recalls Steele. "I was at
    the keyboard, and he was at my elbow," says Steele.
    "He was perfectly willing to let me type, but he was
    also telling me what to type. 

    The programming session lasted 10 hours. Throughout
    that entire time, Steele says, neither he nor Stallman
    took a break or made any small talk. By the end of the
    session, they had managed to hack the pretty print
    source code to just under 100 lines. "My fingers were
    on the keyboard the whole time," Steele recalls, "but
    it felt like both of our ideas were flowing onto the
    screen. He told me what to type, and I typed it." 

   The length of the session revealed itself when Steele
   finally left the AI Lab. Standing outside the building
   at 545 Tech Square, he was surprised to find himself
   surrounded by nighttime darkness. As a programmer,
   Steele was used to marathon coding sessions. Still,
   something about this session was different. Working with
   Stallman had forced Steele to block out all external
   stimuli and focus his entire mental energies on the task
   at hand. Looking back, Steele says he found the Stallman
   mind-meld both exhilarating and scary at the same
   time. "My first thought afterward was: it was a great
   experience, very intense, and that I never wanted to do
   it again in  my life."  - Guy Steele

Heck! to get a taste one can take a stab at writing a formatter for printing a floating point number.