elseif keyword in if-then-else

## Description

If there is more than one condition then keyword `elseif` is useful. If there is more than one `elseif` then the first true condition is selected (even other also are true).

## Examples

    a=1;
    if a==1 then
        disp("a equals 1")
    elseif a==2 then
        disp("a equals 2")
    elseif a==1 & a>0 then
        disp("this condition is true, but never will be selected, because it is the second true condition.")
    else
        disp("Case not handled.")
    end

## See also

- if
- select

## History

6.0.0 `elseif` is now protected: Assignments like `elseif=1` are no longer possible.