P:(T
), a:array(T), i:
|a|, v:T. array-count(v.P(v);a[i:=v]) = array-count(v.P(v);a)+if P(v)
if P(a[i]) 0 else 1 fi ;P(a[i])-1 else 0 fin:, f:(n
), m:n. sum(f(x) | x < n) = f(m)+sum(if x=
m 0 else f(x) fi | x < n)a:array(T), j,i:|a|, v:T. a[i:=v][j] ~ if j=i v else a[j] fin:, f,g:(n). (i:n. f(i) = g(i)) 
sum(f(x) | x < n) = sum(g(x) | x < n)