virtual void build_ligature (Spanner *ligature, vector<Grob_info> primitives);
virtual void transform_heads (Spanner *ligature,
vector<Grob_info> primitives) = 0;
virtual void build_ligature (Spanner *ligature, vector<Grob_info> primitives);
virtual void transform_heads (Spanner *ligature,
vector<Grob_info> primitives) = 0;