code: @NonNull
      public static VariableExpression variable(@NonNull String name) {
          if (name == null) {
              throw new IllegalArgumentException("name cannot be null.");
          }
          return new VariableExpression(name);
      }
docs: Creates a variable expression. The variable are used to represent each item in an array
  in the quantified operators (ANY/ANY AND EVERY/EVERY <variable name> IN <expr> SATISFIES <expr>)
  to evaluate expressions over an array.
  
  @param name The variable name
  @return A variable expression