Why is "export default var a = 1;" invalid syntax?

From people.mozilla.org/~jorendorff/es6-draft.html#sec-exports, it seems that export default var a = 1; is not valid in es6. I wonder why that’s the case. You can already do var a = 1;export default a;. Why not make export default var a = 1; valid? Maybe I missed something. Could someone lighten me? Exporting a variable declaration list as "default" wouldn't make sense: for one, the list can have

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ES6 modules クイズ export defaultとVariableDeclaration