func[] bnmul a[] b[] .
   len r[] len a[] + len b[]
   for ia = 1 to len a[]
      h = 0
      for ib = 1 to len b[]
         h += r[ia + ib - 1] + b[ib] * a[ia]
         r[ia + ib - 1] = h mod 10000000
         h = h div 10000000
      .
      r[ia + ib - 1] += h
   .
   while r[$] = 0
      len r[] -1
   .
   return r[]
.
func$ str bn[] .
   s$ = bn[$]
   for i = len bn[] - 1 downto 1
      h$ = bn[i]
      s$ &= substr "0000000" 1 (7 - len h$) & h$
   .
   return s$
.
func[] bn s$ .
   i = len s$ - 7 + 1
   while i >= -5
      r[] &= number substr s$ i 7
      i -= 7
   .
   return r[]
.
print str bnmul bn "18446744073709551616" bn "18446744073709551616"