code: public ExpressionBuilder functions(Function... functions) {
          for (Function f : functions) {
              this.userFunctions.put(f.getName(), f);
          }
          return this;
      }
docs: Add multiple {@link net.objecthunter.exp4j.function.Function} implementations available for use in the expression
  @param functions the custom {@link net.objecthunter.exp4j.function.Function} implementations
  @return the ExpressionBuilder instance