202E; RIGHT-TO-LEFT OVERRIDE
   206A; INHIBIT SYMMETRIC SWAPPING
   206B; ACTIVATE SYMMETRIC SWAPPING
   206C; INHIBIT ARABIC FORM SHAPING
   206D; ACTIVATE ARABIC FORM SHAPING
   206E; NATIONAL DIGIT SHAPES

    assertThat(stringPrep("\u200e")).isNull() // LEFT-TO-RIGHT MARK
    assertThat(stringPrep("\u206e")).isNull() // NATIONAL DIGIT SHAPES
    assertThat(stringPrep("\u206f")).isNull() // NOMINAL DIGIT SHAPES
  }

  @Test fun prohibitionTaggingCharacters() {
    assertThat(stringPrep("\udb40\udc01")).isNull() // Note that this is one code point.

2066..2069    ; disallowed                             # 6.3  LEFT-TO-RIGHT ISOLATE..POP DIRECTIONAL ISOLATE
206A..206F    ; disallowed                             # 1.1  INHIBIT SYMMETRIC SWAPPING..NOMINAL DIGIT SHAPES
2070          ; mapped                 ; 0030          # 1.1  SUPERSCRIPT ZERO
2071          ; mapped                 ; 0069          # 3.2  SUPERSCRIPT LATIN SMALL LETTER I