code: @Nonnull
      Operation parseParameterList() {
          Expression left = null;
          char ch;
          do {
              nesting++;
              Expression expr = parseExpression( (char)0 );
              nesting--;
              left = concat( left, ';', expr );
              ch = read();
          } while( ch == ';' );
          if( ch != ')' ) {
              throw createException( "Unrecognized input: '" + ch + "'" );
          }
          if( left == null ) {
              return new Operation( reader );
          }
          if( left.getClass() == Operation.class ) {
              switch( ((Operation)left).getOperator() ) {
                  case ',':
                  case ';':
                      return (Operation)left;
              }
          }
          return new Operation( reader, left, ',' );
      }
docs: Parse a parameter list for a function.
  
  @return the operation