-- foo.cue --
#MyEnum:
    #Val1 |
    #Val2 |
    #Val3

#Val1: 1
#Val2: 2
#Val3: 3

a0: #MyEnum
a1: [...#MyEnum]

-- input.textproto --
a0: Val1
a1: Val1
a1: Val2
a1: Val3
a1: 2

-- out/decode --
a0: 1
a1: [1, 2, 3, 2]