There are well-known rules for indenting Lisp code, but it's sometimes convenient to break the rules when the code has semantic structure not reflected in the S-expressions. This sometimes happens when there are alternating sequences, as in some versions of let and cond and setf . It's also common to leave forms unindented when they're supposed to be top-level but something has been wrapped around them anyway, such as eval-when , or the library or unit forms required in some Scheme module systems. In either case, the indentation reflects the structure a human reader understands the program by, rather than the structure of its S-expressions.

JRM recently posted a particularly extreme example of irregular indentation:

(define (polynomial-erf z) ;; Numerical Recipies 6.2 ;; fractional error < 1.2e-7 (let* ((t (/ 1.0 (+ 1.0 (* .5 (abs z))))) (ans (- 1.0 (* t (exp (+ (- (* z z)) (+ -1.26551223 (* t (+ 1.00002368 (* t (+ .37409196 (* t (+ .09678418 (* t (+ -.18628806 (* t (+ .27886807 (* t (+ -1.13520398 (* t (+ 1.48851587 (* t (+ -.82215223 (* t .17087277)))))))))))))))))))))))) (if (>= z 0) ans (- ans))))

Note the big polynomial written in sequential multiply-and-add style. It has two semantic structures: it's a tree of arithmetic expressions, but it's also a sequence of polynomial terms. The latter interpretation is more useful, and the indentation reflects this: it's intended to be read as a sequence, so it's indented as a sequence. If it were indented normally, according to its tree structure, its semantic sequentiality would be less obvious, not to mention it would be impractically wide:

(+ -1.26551223 (* t (+ 1.00002368 (* t (+ .37409196 (* t (+ .09678418 (* t (+ -.18628806 (* t (+ .27886807 (* t (+ -1.13520398 (* t (+ 1.48851587 (* t (+ -.82215223 (* t .17087277))))))))))))))))))

Either way, that's an imposing pile of right-parens. (You could avoid this with a simple polynomial function or macro, but it's not worth the trouble for one polynomial.)

One of the reasons expression-based languages are so convenient is that their program structure reflects the semantic structure humans understand them by. Irregular indentation is a reminder that they don't always do so perfectly.