Trim() function (and trim_left/trim_right) has state that affects next invocations

[hell-racer]

Consider the following test:

[Fact]
public void TrimTest()
{
    const string json = "{\"a\": \"b c\"}";
    const string expression = "trim(a, 'c') | trim(@)";
    const string expected = "b";

    Assert(expression, json, expected);
}

This fails because the second argument (characters) is saved into local field and then used for the next invocations. If I change expression to trim(a, 'c') | trim(@, ' '), the test passes.