flowchart LR
  %% element use
  T --> C1 --> C11 --> O111
  
  %% element definition
  E([ exp ])
  C([ com ])
  T([ the ])
  
  C1(( QM ))
  C11(( MNDO ))
  O111[\ DR1 \]
  C2(( MD ))
  C3(( FD ))
  C4(( SVFE ))
  
  %% class use
  class E approach
  class C approach
  class T approach
  class C1 topic
  class C11 topic
  class O111 output
  
  %% class style definition
  classDef approach fill:#afa,stroke:#3a3,stroke-width:2px
  classDef topic fill:#aaf,stroke:#33a,stroke-width:2px
  classDef output fill:#faa,stroke:#a33,stroke-width:2px