code: public static String strip(final String str, final String stripChars) {
          if (N.isNullOrEmpty(str)) {
              return str;
          }
  
          return stripEnd(stripStart(str, stripChars), stripChars);
      }
docs: <p>
  Strips any of a set of characters from the start and end of a String.
  This is similar to {@link String#trim()} but allows the characters to be
  stripped to be controlled.
  </p>
  
  <p>
  A {@code null} input String returns {@code null}. An empty string ("")
  input returns the empty string.
  </p>
  
  <p>
  If the stripChars String is {@code null}, whitespace is stripped as
  defined by {@link Character#isWhitespace(char)}. Alternatively use
  {@link #strip(String)}.
  </p>
  
  <pre>
  N.strip(null, *)          = null
  N.strip("", *)            = ""
  N.strip("abc", null)      = "abc"
  N.strip("  abc", null)    = "abc"
  N.strip("abc  ", null)    = "abc"
  N.strip(" abc ", null)    = "abc"
  N.strip("  abcyx", "xyz") = "  abc"
  </pre>
  
  @param str
  the String to remove characters from, may be null
  @param stripChars
  the characters to remove, null treated as whitespace
  @return the stripped String, {@code null} if null String input