seq(a,b,n) returns a + t*(-b+a) where t is
numeric vector seq(from=0,to=1,len=n).
This definition is one of several plausible alternatives, but has the
nice property that the first and last elements are exactly equal to
a and b respectively.
a <- as.3vel(c(4,5,6)/9)
b <- as.3vel(c(-5,6,8)/14)
x <- seq(a,b,len=9)
x[1]-a # should be zerox[9]-b # should be zerojj <- a + seq(0,1,len=9)*(b-a)
jj-x # decidedly non-zero