Function interp_gtree 

fn interp_gtree(
    args: &[RValue],
    named: &[(String, RValue)],
    context: &BuiltinContext<'_>,
) -> Result<RValue, RError>

Create a gTree (group of grobs).

@param children list of grobs (gList) @param name unique grob name @param gp graphical parameters @param vp viewport @return a gTree grob object @namespace grid