private AstItem ReadWhileLoop(TokenIterator tokens)
{
//while a > 5 {...}
tokens.PopExpected(TokenKind.While);
var whileExpression = ReadExpression(tokens, TokenKind.CurlyBracketOpen);
tokens.PopExpected(TokenKind.CurlyBracketOpen);
var statements = ReadStatements(tokens);
tokens.PopExpected(TokenKind.CurlyBracketClose);
return(AstItem.WhileLoop(whileExpression, statements));
}