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).
prefixes(x, i)A tracks object is returned which contains all the subtracks of any track in the input tracks object that consist of exactly `i` segments and start at the first registered coordinate of the given track.
a single track or a tracks object.
subtrack length. A single integer, lists are not supported.
This function behaves exactly like subtracks except
that only subtracks starting from the first position are considered.
subtracks to extract all subtracks of a given length,
subtracksByTime to extract all subtracks of a given length
starting at some fixed timepoint, and selectSteps to extract
single steps starting at a fixed timepoint from a subset of trackids.