When you enter a piece of input such as 2+2, Mathematica first recognizes the + as an operator and constructs the expression Plus[2,2], then uses the built-in rules for Plus ...
AiryBiPrime[z] gives the derivative of the Airy function Bi^\[Prime] (z).
JacobiCS[u, m] gives the Jacobi elliptic function cs(u | m).
JacobiDS[u, m] gives the Jacobi elliptic function ds(u | m).
JacobiSD[u, m] gives the Jacobi elliptic function sd(u | m).
NevilleThetaD[z, m] gives the Neville theta function \[CurlyTheta]_d (z \[VerticalSeparator] m).
NevilleThetaN[z, m] gives the Neville theta function \[CurlyTheta]_n (z \[VerticalSeparator] m).
NevilleThetaS[z, m] gives the Neville theta function \[CurlyTheta]_s (z \[VerticalSeparator] m).
SinIntegral[z] gives the sine integral function Si(z).
WeierstrassSigma[u, {g_2, g_3}] gives the Weierstrass sigma function \[Sigma](u; g_2, g_3).