title: "clean" not working in make file
body: <p>This is my make file.</p>
  
  <pre><code> all: observer
  
      observer: main.o weather_center.o display.o subject.o observer.o
           g++ main.o weather_center.o display.o subject.o observer.o -o observer
  
      main.o: main.cpp
          g++ -c main.cpp
  
      weather_center.o: weather_center.cpp
          g++ -c weather_center.cpp
  
      display.o: display.cpp
          g++ -c display.cpp
  
      subject.o: subject.cpp
          g++ -c subject.cpp
  
      observer.o: observer.cpp
          g++ -c observer.cpp
  
      clean:
          rm -f  *o observer
  </code></pre>
  
  <p>Here I'm trying to use </p>
  
  <pre><code>clean:
          rm -f  *o observer
  </code></pre>
  
  <p>To clean up the temporary *.o files. But program compiles and generate the target assembly, but doesn't delete the *.o files. Not showing any errors also.</p>
  
  <ul>
  <li>I tried <code>rm -f  *o observer</code> in terminal. It works fine.</li>
  <li>I have used <code>Tab</code> for indent</li>
  <li>there are no files start with <code>clean</code> or <code>rm</code> in the directory.</li>
  <li>tried <code>$(RM)</code> instead of <code>rm</code>. but no lucky</li>
  </ul>