12 line program... `main` is a nested function when trying to use redblacktree. Beginner error???

[WhatMeWorry`]

import std.container : RedBlackTree;

int main()
{

     struct Location {
         int x;
         int y;
     }

     struct Node{
         this(Location loc, uint f) {
             this.loc = loc;
             this.f = f;
         }
         Location loc;
         uint f;
     }

auto priorityQueue = new RedBlackTree!(Node, "a.f < b.f", true); 
// true: allowDuplicates

// 
C:\D\dmd2\windows\bin64\..\..\src\phobos\std\container\rbtree.d(806): Error: `main` is
// a nested function and cannot be accessed from
// `std.container.rbtree.RedBlackTree!(Node, "a.f < b.f", 
true).RedBlackTree.allocate`

return 0;
}