A special function for composing
When we combine things with addition, we call 0 the "additive identity" since 0+𝑥=𝑥 and 𝑥+0=𝑥 
When we combine things with multiplication, we call 1 the "multiplicative identity" since 1⋅𝑥=𝑥 and 𝑥⋅1=𝑥 
When we combine functions with composition, we call 𝑓𝑥=𝑥 the "identity function" since 𝑓𝑔𝑥=𝑔𝑥 and 𝑔𝑓𝑥=𝑔𝑥.