adaptivity_options.rnc produces invalid relax-rng schema

A long way down in adaptivity_options.rnc is the following definition:
element format {
    "triangle",
    comment
}

This produces an invalid RNG file, since elements cannot contain data and string children.

Minimal test case:

$ cat > foo.rnc <<\EOF
start =
   (
      element format {
         "triangle",
         element comment {
            xsd:string
         }?

      }
   )
EOF

$ spud-preprocess foo.rnc
$ jing foo.rng
foo.rng:4:28: error: group of "string" or "data" element

xmllint also complains

$ xmllint --relaxng foo.rng >/dev/null
foo.rng:4: element element: Relax-NG parser error : Element format has a content type error
Relax-NG schema foo.rng failed to compile

I'm not sure what the correct definition here was meant to be, maybe:

      element format {
         attribute name {"triangle"},
         element comment {
            xsd:string
         }?