Therefore not all letters are allowed. The solution adopted in calc101 is to only allow two kinds of functions. Parameters are also restricted to be a single letter. Avoid d, e, m, n, u, v, which are used by calc101 and C, D, E, I, K, N, O which have a meaning in Mathematica . D means the derivative operator.
E is the base of the natural logarithms.
I is the square root of minus 1. K is used in certain summations as an indexed constant. O is used in series as the big-oh notation. That's because there may be a choice of different methods at various points. If the problem is simple enough, the answers will usually look the same.