Extract function(s) from a functional sequence.
Functional sequences can be subset using single or double brackets. A single-bracket subset results in a new functional sequence, and a double-bracket subset results in a single function.
# S3 method for fseq [[(x, ...)
# S3 method for fseq [(x, ...)
A functional sequence
index/indices. For double brackets, the index must be of length 1.
A function or functional sequence.