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That means only the first letters are compared in a case-sensitive manner. Other letters are compared case-insensitively within the ASCII range and underscores are ignored.
This rather unorthodox way to do identifier comparisons is called partial case-insensitivity and has some advantages over the conventional case sensitivity:
It allows programmers to mostly use their own preferred spelling style, be it humpStyle or snake_style, and libraries written by different programmers cannot use incompatible conventions. A Nim-aware editor or IDE can show the identifiers as preferred. Another advantage is that it frees the programmer from remembering the exact spelling of an identifier. The exception with respect to the first letter allows common code like var foo: Foo to be parsed unambiguously.