Difference between robbery-theft and robbery-extortion

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a robbery would always be either theft or extortion as shown by the definition, in practice it may sometimes be quite difficult to identify as to which part is robbery by theft and which one robbery by extortion. For instance, A enters into the house of В and pointing a revolver at him asks him to surrender all the valuables. While В starts surrendering the valuables, A himself starts picking up some of the other valuables.