CAS bug when calculating sum(1/n,n,1,medium_value)

[komame]

Additionally, I noticed another issue.

To reproduce, run it on a real G1 or G2:

• perform ON+Symb (soft reset to clear junk from memory)
• enter CAS
• input the sum for (1/x) from x=1 to 2000 and execute for the numeric value using Shift+Enter => the result will appear within about 1 second;
• change the range: from 1 to 2000000 and run again => despite the larger range, the result will appear immediately;
• restore the range from 1 to 2000, and run again => this time you'll wait for the result for several minutes...
  

It doesn't matter whether it's CAS or not. Such behavior is non-standard, and surely something can be improved here to ensure that a previous operation doesn't affect the execution time of subsequent ones.