As seen in the documentation, the keyword
ref has several forms.
Single-line statement identified by a colon:
ref over Server: LISTEN
Or as multi-line statements in the following format:
ref over K, K
Kim makes a sandwich
It also supports a form on inline syntax with other WSD-syntax:
Alice->ref over Bob, Mary: input
refer to other diagram
end ref -->Alice: output
wsd-mode only supports the first two syntaxes, and not the third.
Not supporting this syntax means that we're not 100% feature complete syntax-wise, but supporting this specific syntax is currently not a priority.
This issue is first and foremost meant to document to self (and others) that this shortcoming exists.