This dataset defines an alphabet for the type "pc_set". It is a list with two components, defining a forward and backward mapping between integers and "pc_set" objects:
by_id:
A list object.
The ith element of this list contains the pitch-class set
corresponding to integer i.
by_chord:
An environment object.
A pitch-class set can be mapped to an integer by
taking the pitch-class set object,
converting it to a string representation (with as.character),
and accessing the object within this environment with this name.
The result will be a coded vector (see coded_vec)
of length 1.