1sink: "{ S[] -> a[] }"
2must_source: "{ T[] -> a[] }"
3may_source: "{ U[] -> a[] }"
4schedule:
5  domain: "{ U[]; S[]; T[] }"
6  child:
7    sequence:
8    - filter: "{ T[]; U[] }"
9    - filter: "{ S[] }"
10