tabs.md

@@ -27,3 +27,23 @@ Cons:
 - more complicated to explain than just disallowing tabs altogether
 - more to implement
 - hard to precicely specify, language syntax extensions would have to be careful not to turn valid programs into invalid one due to this rule.
+
+## Concrete implementation idea for the above
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+
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+leading tabs are tabs that precede the first non-whitespace character on a line.
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+if leading tabs are present, all leading whitespace must be made up of tabs.
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+Allow leading tabs only if the current line establishes a new indentation level and all enclosing indentation levels were established with tabs. You must use leading tabs to continue a tab established indentation level, even when you are not adding a new statement but rather continuing the previous line.
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+any line that contains tabs in a non-leading position may not establish a new indentation level. (as in, the body of a let,where, etc must all be on the current line or start on the next line.)
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+