Backslash is swallowed when unescaping sequence like `\E\R\`

[fathom-piotr]

See this example piece of code:

msg = hl7.Message(separator="\r", sequence=[["MSH", "^~\\&"]])
unescaped = msg.unescape("\\E\\R\\")

unescaped should be \R\ because:

• the first \E\ becomes a literal \
• we're left with R\

The library, however, returns \R, as if the trailing slash could be skipped.

[fathom-piotr]

After a bit of digging, there's a question if HL7 defines what happens with an open escaping sequence. The library skips whatever is after the last \ because it in fact collects anything after it hoping to get an escaping sequence, but the closing \ never appears, so whatever is collected is lost. If HL7 doesn't define the expected behavior, my take on this is that such a value is broken (not well-formed) and the intended behavior is undefined.

Going forward with this approach, if we want to encode \R\, we should have:

\E\R\E\
^^^ ^^^
 |   |
 |   \- second escaping sequence producing '\'
 \------ first escape sequence producing '\'