about abstraction of libc procedures

[moliam]

In angr, the calls into libc procedures such as atoi, strcmp, strlen, etc, are abstracted and simulated with expressions consisting of series of "if" and "else". Is this abstraction work difficult to realize or does it require much intelligence?
By the way, I did some experiments with angr and it seems that for "strcmp", angr can only solve the situation where the condition is like 0 == strcmp(a, b) or 0 != strcmp(a, b), but cannot solve 0 > strcmp(a, b) or 0 < strcmp(a, b). Is this due to the incompleteness of the procedure abstraction model?

Thank you very much!