maint: Clean up m-files to follow Octave coding conventions. Try to trim long lines to < 80 chars. Use '##' for single line comments. Use '(...)' around tests for if/elseif/switch/while. Abut cell indexing operator '{' next to variable. Abut array indexing operator '(' next to variable. Use space between negation operator '!' and following expression. Use two newlines between endfunction and start of %!test or %!demo code. Remove unnecessary parens grouping between short-circuit operators. Remove stray extra spaces (typos) between variables and assignment operators. Remove stray extra spaces from ends of lines.

--- a/scripts/strings/substr.m
+++ b/scripts/strings/substr.m
@@ -99,7 +99,7 @@
 %!assert (substr ("This is a test string", 1, -7), "This is a test")
 %!assert (isempty (substr ("This is a test string", 1, 0)))
 
-%% Test input validation
+## Test input validation
 %!error substr ()
 %!error substr ("foo", 2, 3, 4)
 %!error substr (ones (5, 1), 1, 1)