function fix(m,F)
   return fn f|n|
      if n not in m then m[n] = F(f,n) end
      return m[n]
   end
end

Lazy = table(Type,"Lazy"){
   function string
      "lazy expression"
   end,
   function value
      if self.x is null
         x = self.f()
         self.x = x
         return x
      else
         return self.x
      end
   end
}

function lazy(f)
   return table Lazy{f=f,x=null}
end

Function.add = |f;g| |x| f(x)+g(x)
Function.sub = |f;g| |x| f(x)-g(x)
Function.mul = |f;g| |x| f(x)*g(x)
Function.div = |f;r| |x| f(x)/r
Function.rmul = |r;f| |x| r*f(x)