collections/string-operator.cfa:283:1 error: Cannot choose between 2 alternatives for expression
Applying untyped:
  Name: ?+?
...to:
  Applying untyped:
    Name: ?+?
  ...to:
    Constant Expression ('a': char)
    ... with resolved type:
      char
    Constant Expression ('b': char)
    ... with resolved type:
      char

  Name: s
 Alternatives are:
Cost ( 0, 0, 0, 0, 0, 0, 0, 0 ): Application of
      Variable Expression: ?+?: function
      ... with parameters
        char
        instance of struct string with body
      ... returning
        instance of struct string with body

      ... with resolved type:
        pointer to function
        ... with parameters
          char
          instance of struct string with body
        ... returning
          instance of struct string with body

      ... to arguments
      Application of
        Variable Expression: ?+?: function
        ... with parameters
          char
          char
        ... returning
          char

        ... with resolved type:
          pointer to function
          ... with parameters
            char
            char
          ... returning
            char

        ... to arguments
        Constant Expression ('a': char)
        ... with resolved type:
          char
        Constant Expression ('b': char)
        ... with resolved type:
          char

      ... with resolved type:
        char
      Variable Expression: s: instance of struct string with body
      ... with resolved type:
        instance of struct string with body

    ... with resolved type:
      instance of struct string with body
  (types:
    instance of struct string with body
  )
  Environment:

Cost ( 0, 0, 0, 0, 0, 0, 0, 0 ): Application of
      Variable Expression: ?+?: function
      ... with parameters
        instance of struct string with body
        instance of struct string with body
      ... returning
        instance of struct string with body

      ... with resolved type:
        pointer to function
        ... with parameters
          instance of struct string with body
          instance of struct string with body
        ... returning
          instance of struct string with body

      ... to arguments
      Application of
        Variable Expression: ?+?: function
        ... with parameters
          char
          char
        ... returning
          instance of struct string with body

        ... with resolved type:
          pointer to function
          ... with parameters
            char
            char
          ... returning
            instance of struct string with body

        ... to arguments
        Constant Expression ('a': char)
        ... with resolved type:
          char
        Constant Expression ('b': char)
        ... with resolved type:
          char

      ... with resolved type:
        instance of struct string with body
      Variable Expression: s: instance of struct string with body
      ... with resolved type:
        instance of struct string with body

    ... with resolved type:
      instance of struct string with body
  (types:
    instance of struct string with body
  )
  Environment:


