{
  type: "CallExpression",
  callee: {
    type: "MemberExpression",
    object: {
      type: "Identifier",
      name: "R"
    },
    property: {
      type: "Identifier",
      name: "merge"
    }
  },
  arguments: [
    {
      type: "Identifier",
      name: "a"
    },
    {
      type: "Identifier",
      name: "b"
    }
  ]
}