warning: suggest parenthesis around assignment used as truth value

[José Abílio Matos]

I am using octave 6 and I am getting this dreadful :-):

warning: suggest parenthesis around assignment used as truth value near line 
135...

The line where it complains is the one after the elseif:

if length(rhs) < pow || isempty(rhs{pow})
    rhs{pow} = terms{j}(start:end);
elseif 
    rhs{pow} = [rhs{pow}, ' + ', terms{j}(start:end)];
end


In terms of context we have that "terms" is a cell string (in column mode) and 
"rhs" is also a cell string.


Searching I found this previous thread:
https://lists.gnu.org/archive/html/octave-maintainers/2009-03/msg00289.html


I am reporting this because I was surprised by the warning and I fail to see 
where is this relevant.

Thank you,
-- 
José Matos