In the event that a calculation appears to be running excessively long, one can abort the
calculation by selecting Evaluation from the Mathematica menu strip and then choosing Abort
Evaluation. A calculation is in progress when the thickness of the cell bracket on the rightmost
edge of the notebook window increases and appears as a thick black vertical line.
原文：http://www.cnblogs.com/goodmangis/p/4204498.html