@acer Ok, thanks! Does it imply that good practice is to always use ~ when I work with a matrix or a vector of symbolic expressions? 

@Carl Love Oh, I see. Do you know maybe a book or a guide focused on such crucial differences? I am working now with various Getting Started documents, but they do not discuss such things. ;)

@Mariusz Iwaniuk 

Thanks a lot! But why does it work? What is the difference between

simplify(sum(v . (Transpose(v)), t = k .. k + 13))

and 

simplify(eval(sum(v . (Transpose(v)), t = k .. k + n), n = 13));

@acer 

Thanks! How does it work? I cannot find in the documentation what does the tilde after a function name do?