> LISP programmers back then read and wrote programs in this form, on paper. Yes, program sources were meant to be read by humans. In order for computers to und

> LISP programmers back then read and wrote programs in this form, on paper. Yes, program sources were meant to be read by humans. In order for computers to understand programs, one must have prepared a deck of cards with holes punched according to BCD character code. The character set didn’t have many punctuations and it was impossible to punch M-expressions directly. Instead, people translated M-expressions into S-expressions, which was already used to describe nested list structures. In other words, people fed the machine the internal representation of syntax tree directly. > Eventually, people got used to write programs in S-expressions directly, and M-expressions were faded into oblivion. Ah, finally a satisfying explanation for this.