Given a cell class create an index representation. If no source is specified no index will be generated.
# S3 method for default
index(object, id = "tangram", name = NULL, key.len = 4, ...)
A list of strings containing key, source and value
cell; The cell for indexing
character; an additional specifier for the object key
character; optional names of elements inside object
numeric; length of generated key
additional arguments to renderer. Unused