A step is a movement from one point to the next, with an sftraj object this manifests as a linestring. If, however, one of these two points is missing, the sftraj is created as a geometery collection of two points, the beginning and the end point, where one of the steps is NA. This function checks a trajectory geometry if its a linestring and returns a vector of T/F.
is_linestring(x)an sftraj object