Creates a tracks object consisting of all prefixes (i.e., subtracks
starting with the first position of a track) of `x`
with `i` segments (i.e., `i`+1 positions).
Usage
prefixes(x, i)
Arguments
x
a single track or a tracks object.
i
subtrack length. A single integer, lists are not supported.
Details
This function behaves exactly like subtracks except
that only subtracks starting from the first position are considered.