function _posEq(pos1, pos2) {
        return pos1 && pos2 && pos1.line === pos2.line && pos1.ch === pos2.ch;
    }
@private
Checks if two CodeMirror-style {line, ch} positions are equal.
@param {{line: number, ch: number}} pos1
@param {{line: number, ch: number}} pos2
@return {boolean} true if pos1 and pos2 are equal. Fails if either of them is falsy.