a(5) <= 23 Boole expansion (Knuth page 52)
XOR costs 3
a(5) <= 20. Modify procedure used to calculate a(5) in A056287 to add XOR.
a(5) <= 18. Let f = g(xi = h), then cost(f) <= cost(h) + cost(g). This is a generalisation of minimum memory operations (xi = xj op xk  Knuth page 101). Add this for min(cost(g),cost(h)) <= 3.
a(5) <= 17. Add cost(f(xi,xj = xi op1 xj, xi op2 xj)) <= cost(f) + 2 (Knuth page 105).
a(5,17) <= 187 where a(5,c) is the number of functions classes that cost c.
a(5,17) <= 1 (David Ian Stevenson).
a(5) >= 13 Stockmeyer limit for symmetric function S124 (Knuth exercise 7.1.2  80 and answer to 20 which clarifies Richard C. Schroeppel's hardest functions).
