Bjørn-Helge Mevik (matlabColon) and Øyvind Langsrud (SeqInc)
Details
matlabColon(a,b) returns a:b (R's version) unless a > b, in which case it returns integer(0).
SeqInc(a,b) is similar, but results in error when the calculated length of the sequence (1+to-from) is negative.