This Commented CUE demonstrates how to use the cue command to combine multiple YAML files into a list.

a: 1
b: "two"
c: 33.3
a: 22
b: "some string"
a: 333
c: 42
package example

import "list"

// _inputs is the location under which we place
// each data file's contents, so that their
// contents can be accessed by the CUE in this
// file. The cue CLI -l parameter places them
// here, under a secondary key taken from the
// file's name.
_inputs: [string]: _

// Each file's contents must adhere to the #Input
// schema.
_inputs: [string]: #Input

#Input: {
	a!: int
	b?: string
	c?: number

// Construct a sorted list of the input files'
// names so that our output (an ordered list) can
// remain stable no matter the order the
// filenames are provided to the cue CLI.
let sortedInputs = list.SortStrings([
	for k, _ in _inputs {k},

// Iterate over the stable input filename list,
// and emit each input file's contents in turn.
contents: [for _name in sortedInputs {
$ cue export --with-context -l '_inputs:' -l filename a.yaml b.yml c.yaml example.cue --out yaml
  - a: 1
    b: two
    c: 33.3
  - a: 22
    b: some string
  - a: 333
    c: 42