docs: function buildFunction( def, code, varName ) {
      if ( isSpecial( def, "behind" ) ) {
          const behindFunctionName = def[ 1 ];
          code.addNeededBehindFunction( behindFunctionName );
          code.that = true;
          return [
              "value => {",
              "  try {",
              `    CODE_BEHIND${keySyntax(behindFunctionName)}.call(that, value, ${varName});`,
              "  }",
              "  catch( ex ) {",
              "    console.error(`Exception in code behind \"" +
              behindFunctionName + "\" of module \"" +
              code.moduleName + "\": ${ex}`);",
              "  }",
              "}"
          ];
      }
  
      if ( isSpecial( def, "toggle" ) ) {
          const nameOfTheBooleanToToggle = def[ 1 ];
          const namesOfTheBooleanToToggle = Array.isArray( nameOfTheBooleanToToggle ) ?
              nameOfTheBooleanToToggle : [ nameOfTheBooleanToToggle ];
          const body = namesOfTheBooleanToToggle
              .map( name => `${getElemVarFromPath(name)} = !${getElemVarFromPath(name)};` );
          return [
              "() => {",
              body,
              "}"
          ];
      }
  
      throw Error( `Function definition expected, but found ${JSON.stringify(def)}!` );
  }