50、计算复杂度：P、NP 与布尔可满足性问题

计算复杂度：P、NP 与布尔可满足性问题

1. 2 - 可满足性问题的公式简化

在解决 2 - 可满足性问题时，公式简化是一个关键步骤。 simplify-formula 函数用于简化公式，它接受一个公式和一个单例作为输入，并返回一个简化后的公式。

;; formula singleton →formula
;; Purpose: Simplify the given formula by removing the given singleton
;; Assumption: The given formula does not have the complement of the given
;; singleton as a clause and the given singleton’s variable is
;; assigned a value that makes the singleton true
(define (simplify-formula a-formula a-clause)
  (define (simplify a-formula)
    (cond [(empty? a-formula)
           '()]
          [(equal? (first a-formula) a-clause)
           (simplify-formula (rest a-formula) a-clause)]
          [(not (2disjunction? (first a-formula)))
           (cons (first