cases ps
ps : List Program
df : DagFormula
⊢ undagDagFormula (⌈⌈ps⌉⌉df) = ⌈⌈ps⌉⌉undag df
case nil
df : DagFormula
⊢ undagDagFormula (⌈⌈[]⌉⌉df) = ⌈⌈[]⌉⌉undag df

case cons
df : DagFormula
head✝ : Program
tail✝ : List Program
⊢ undagDagFormula (⌈⌈head✝ :: tail✝⌉⌉df) = ⌈⌈head✝ :: tail✝⌉⌉undag df