private AstItem ReadVariableAssignment(TokenIterator tokens)
{
var name = tokens.PopExpected(TokenKind.Word);
tokens.PopExpected(TokenKind.AssigmnentOperator);
var expression = ReadExpression(tokens, TokenKind.SemiColon);
tokens.PopExpected(TokenKind.SemiColon);
return(AstItem.VariableAssignment(name.Value, expression));
}