The instance of AList for the input.
The instance of AList for the input. For a representationC of [L[x]], this Tree should have a Type of AList[L]
Returns the completed value containing all expressions added to the builder.
Represents the higher-order type constructor [L[x]] ... where ... is the
type of the data structure containing the added expressions,
except that it is abstracted over the type constructor applied to each heterogeneous part of the type .