For each absolute value expression, make sure that a matching pair of vertical bars are used.
When vertical bars are used to denote absolute value, this calculator is not designed to handle
the case of absolute value expression nested inside another absolute value expression because it's too
ambiguous to interpret user's intention. For a function like f(x) = | 2 + |x + 3| - x |, use "Abs".
f(x) = | 2 + |x + 3| - x | can be input as f(x) = Abs(2 + Abs(x+3) - x).