The vector generator operator, ':', may only be used when indexing a variable,
inside brackets when creating a matrix, or in the argument list of a function.

[e1:e2]  creates the row vector [e1, e1+1, e1+2, ..., e2] if e2 > e1, or
[e1, e1-1, e1-2, ..., e2] if e1 > e2.  e1 and e2 must be scalars.

A colon passed as an argument to a function passes a NULL, and can be used for
arguments which would normally be created by a function, but whose value you
do not want, e.g.  svd(a,:,s) returns the rank of A and creates the matrix s
of singular values, but does not create u or v.