class BiDiGenerate::Schema

can serialize null rather than a fixed tag that can only ever emit the literal.
settable value (the literal to set, null to clear), so it stays a normal field that
`literal | null` (browsingContext.setBypassCSP, emulation.setScriptingEnabled) is a
A const field is a baked discriminator tag, unless it is also nullable: the spec's

def baked_discriminator?(field)
  field['type'].key?('const') && !field['type']['nullable']
end