map(http://example.com/mountpoint) not working as expected

I have an app that's packaged up into a mach stack. I'd like to test variations on it, such that:

http://example1.local:8080/ <- default app
http://example1.local:8080/a <- default app again
http://example1.local:8080/b <- variant B
http://example1.local:8080/c <- variant C

After seeing that this appears to be supported in the docs, I tried each of these techniques:

var mapper = mach.mapper();

mapper.map(
  "http://example1.local",
  stack1
)

mapper.map(
  "http://example2.local",
  stack2
)

mapper.map(
  "http://example3.local",
  stack3
)

mapper.map(
  "http://example1.local/b",
  stack1b
)

mapper.map(
  "http://example2.local/b",
  stack2b
)

mapper.map(
  "http://example3.local/b",
  stack3b
)

mach.server(
  mapper,
  8080
)

var mapper = mach.mapper();

stack1.serve(
  "*",
  // …
)

stack1.map(
  "/b",
  stack1b
)

mapper.map(
  "http://example1.local",
  stack1
)

stack2.serve(
  "*",
  // …
)

stack2.map(
  "/b",
  stack2b
)

mapper.map(
  "http://example2.local",
  stack2
)

stack3.serve(
  "*",
  // …
)

stack3.map(
  "/b",
  stack3b
)

mapper.map(
  "http://example3.local",
  stack3
)

mach.server(
  mapper,
  8080
)

(This one was kind of a mistake. The serve call is in a different file, so I didn't realize I had placed a serve call higher in the stack that the map call.)

var mapper = mach.mapper();

stack1.map(
  "/b",
  stack1b
)

stack1.serve(
  "*",
  // …
)

mapper.map(
  "http://example1.local",
  stack1
)

stack2.map(
  "/b",
  stack2b
)

stack2.serve(
  "*",
  // …
)

mapper.map(
  "http://example2.local",
  stack2
)

stack3.map(
  "/b",
  stack3b
)

stack3.serve(
  "*",
  // …
)

mapper.map(
  "http://example3.local",
  stack3
)

mach.server(
  mapper,
  8080
)

Each time, I received an error like this:

Warning: No route matches path "/b". Make sure you have <Route path="/b"> somewhere in your routes

which tells me that the variant isn't being mapped as requested.

mjackson / mach

map(http://example.com/mountpoint) not working as expected #54