beginner tutorials msg srv catkin_make error

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Hey, I'm doing the srv/msg tutorial ( http://wiki.ros.org/ROS/Tutorials/Cre... ), and I'm having an error when i use catkin_make, after changing the CMakeLists.txt and package.xml and generally following the tutorial (and the previous tutorials).

I'm totally new to ROS, and It'll be great if someone can help me out, because I'm pretty lost. After having this error I even tried deleting my progress and recreating it, only to get this same error message again.

Here's my error:

sophie@sophie-HP-ProBook-4520s:~/catkin_ws$ catkin_make
Base path: /home/sophie/catkin_ws
Source space: /home/sophie/catkin_ws/src
Build space: /home/sophie/catkin_ws/build
Devel space: /home/sophie/catkin_ws/devel
Install space: /home/sophie/catkin_ws/install
####
#### Running command: "make cmake_check_build_system" in "/home/sophie/catkin_ws/build"
####
-- Using CATKIN_DEVEL_PREFIX: /home/sophie/catkin_ws/devel
-- Using CMAKE_PREFIX_PATH: /home/sophie/catkin_ws/devel;/opt/ros/indigo
-- This workspace overlays: /home/sophie/catkin_ws/devel;/opt/ros/indigo
-- Using PYTHON_EXECUTABLE: /usr/bin/python
-- Using Debian Python package layout
-- Using empy: /usr/bin/empy
-- Using CATKIN_ENABLE_TESTING: ON
-- Call enable_testing()
-- Using CATKIN_TEST_RESULTS_DIR: /home/sophie/catkin_ws/build/test_results
-- Found gtest sources under '/usr/src/gtest': gtests will be built
-- Using Python nosetests: /usr/bin/nosetests-2.7
-- catkin 0.6.16
-- BUILD_SHARED_LIBS is on
WARNING: Package "ompl" does not follow the version conventions. It should not contain leading zeros (unless the number is 0).
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- ~~  traversing 1 packages in topological order:
-- ~~  - beginner_tutorials
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- +++ processing catkin package: 'beginner_tutorials'
-- ==> add_subdirectory(beginner_tutorials)
-- Using these message generators: gencpp;genlisp;genpy
-- beginner_tutorials: 1 messages, 1 services
-- BUILD_SHARED_LIBS is on
WARNING: Package "ompl" does not follow the version conventions. It should not contain leading zeros (unless the number is 0).
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- ~~  traversing 1 packages in topological order:
-- ~~  - beginner_tutorials
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- +++ processing catkin package: 'beginner_tutorials'
-- ==> add_subdirectory(beginner_tutorials)
CMake Error at /opt/ros/indigo/share/catkin/cmake/catkin_workspace.cmake:116 (add_subdirectory):
  The binary directory

    /home/sophie/catkin_ws/build/beginner_tutorials

  is already used to build a source directory.  It cannot be used to build
  source directory

    /home/sophie/catkin_ws/src/beginner_tutorials

  Specify a unique binary directory name.
Call Stack (most recent call first):
  CMakeLists.txt:125 (catkin_workspace)


-- Configuring incomplete, errors occurred!
See also "/home/sophie/catkin_ws/build/CMakeFiles/CMakeOutput.log".
See also "/home/sophie/catkin_ws/build/CMakeFiles/CMakeError.log".
make: *** [cmake_check_build_system] Error 1
Invoking "make cmake_check_build_system" failed

Also, I added these two lines to my package.xml:

  <build_depend>message_generation</build_depend>
  <run_depend>message_runtime</run_depend>

And my CMakeLists.txt looks like this:

(It's mostly comments. All according to the tutorials, I hope)

cmake_minimum_required(VERSION 2.8.3)
project(beginner_tutorials)

## Find catkin macros and libraries
## if COMPONENTS list like find_package(catkin REQUIRED COMPONENTS xyz)
## is used, also find other catkin packages
find_package(catkin REQUIRED COMPONENTS
  roscpp
  rospy
  std_msgs
  message_generation
)

## System dependencies are found with CMake's conventions
# find_package(Boost REQUIRED COMPONENTS system)


## Uncomment this if the package has a setup.py. This macro ensures
## modules and global scripts declared therein get installed
## See http://ros.org/doc/api/catkin/html/user_guide/setup_dot_py.html
# catkin_python_setup()

################################################
## Declare ROS messages, services and actions ##
################################################

## To declare ...