tsearchn.m: Use Octave coding conventions. * tsearchn.m: Use function name in error() messages. Use "endif" rather than bare "end". Capitalize function input parameters in error() messages to match documentation. Cuddle parentheses to variable when performing indexing. Use '!' for logical not operator. Use two newlines between end of function and start of BIST tests. Add BIST tests for input validation.

%!function out = bug54490 ()
%!  global k;
%!  k = 1;
%!  out = 3;
%!endfunction

%!test <*54490>
%! global k;
%! k = 2;
%! a = [5, 6];
%! a(k) = bug54490 ();
%! assert (a, [5, 3]);
%! k = 2;
%! a = [5, 6];
%! [a(k)] = bug54490 ();
%! assert (a, [5, 3]);
%! clear -global k;  # cleanup after test