Eva (and W.Z.) is right. We need to distinguish between dependent and independent intervals. In the formula for resistance $$R_1$$ and $$R_2$$ are independent intervals (they correspond to resistance of physically different resistors), and in procedure par1 two occurrences of $$R_1$$ are dependent intervals. When we calculate the ratio $$A/A$$ we cannot suppose that those two intervals $$A$$ are independent, we cannot choose their values independently. If we randomly choose the value inside the interval $$A$$, we must assign this value to both occurrences of $$A$$ in the formula. For independent intervals the situation is different, we may choose one value inside $$A$$, some other value inside $$B$$, and their ratio will have meaning, namely this will be some value inside the interval which is the ratio of two intervals, $$A/B$$. Alyssa's package treats all occurrences of the same interval in a complex formula as independent intervals and the answers are wrong. The calculation of $$A/A$$ by Alyssa's package gives wrong result.