Forcing Backtracking

miles(sketches_of_spain)
miles(a_kind_of_blue)

• If Prolog is asked
  miles( X)
  
  it will answer with :
  X = sketches_of_spain
• However, we may be interested in obtaining alternate answers
• To obtain another answer, we force Prolog to invoke backtracking
• Prolog continues execution by failing the last goal that was satisfied and by backtracking as in normal execution - to find another clause whose head matches goal
• Hence :
  miles( X)
  it will answer with :
  X = sketches_of_spain
  X = a_kind_of_blue
  no