public static IndexSpan ToIndexSpan(this SourceSpan span, PythonAst ast)
 => IndexSpan.FromBounds(ast.LocationToIndex(span.Start), ast.LocationToIndex(span.End));
 public static IndexSpan ToIndexSpan(this Range range, PythonAst ast)
 => IndexSpan.FromBounds(ast.LocationToIndex(range.start), ast.LocationToIndex(range.end));
 public static int ToIndex(this SourceLocation location, PythonAst ast) => ast.LocationToIndex(location);