flowchart LR
  C["city[y*128+x]
one byte"]
  C -->|"bits 0-5"| R["Renderer
which cubes to stamp"]
  C -->|"bits 0-5"| T["Terrain collision
can I stand / walk / fall?"]
  C -->|"bit 7"| E["Entity collision
is someone already here?"]
  class C prim