function documentFragment(children, height, depth, maxFontSize) {
                    this.children = children || [];
                    this.height = height || 0;
                    this.depth = depth || 0;
                    this.maxFontSize = maxFontSize || 0;
                }
This file contains metrics regarding fonts and individual symbols. The sigma
and xi variables, as well as the metricMap map contain data extracted from
TeX, TeX font metrics, and the TTF files. These data are then exposed via the
`metrics` variable and the getCharacterMetrics function.