This function returns the object identifier of the object containing the cursor. For singleton tags, this function returns the object identifier of the singleton immediately to the left of the cursor. If the cursor is outside the outermost element or if the document has no objects (that is, an ASCII file or an empty document), this function returns an invalid OID (that is, oid_null). The oid_caret takes an optional argument, doc, that specifies the identifier of the document tree. If doc is omitted or is 0, then the current document is used.