Allow \| to represent a vertical bar in an identifier enclosed in vertical bars.
Pro: It would be more consistent to allow \ to escape the | so that || acts exactly analogously to "" in strings, where backslashes escape the delimiter. This is simpler, more regular and allows implementation to use the same routine for reading strings and symbols (with the delimiter as parameter).
Con: If | is not allowed anywhere in a |-escaped identifier except at the end, we can find the end just by scanning for |.
The WG voted to adopt a proposal in which \| and all the escape sequences valid in strings are valid in identifiers enclosed in vertical bars.