Some of the string functions can only be used with E-strings.
Generally, these are the ones that might extend the string.
If you use a normal string instead you can run into some serious (but subtle) problems.
Commonly misused functions are
Similar problems can arise by using a list when an E-list is required by a list function.
String constants and normal lists are static data, so you shouldn't try to alter their contents unless you know what you're doing (see 9.5.7 Static data).
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