This dataset defines an alphabet for the type "pc_chord". It is a list with two components, defining a forward and backward mapping between integers and "pc_chord" objects:
by_id:
A list object.
The ith element of this list contains the pitch-class chord
corresponding to integer i.
by_chord:
An environment object.
A pitch-class chord can be mapped to an integer by
taking the pitch-class chord 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.