Compound operator with built-in evaluation rules.
f x is by default interpreted as Integrate[f, x].
f x is by default interpreted as Integral[f, x, a, b]. a and b must appear as a subscript and superscript, respectively.
a b x is by default output as (a b) x whenever is an operator with a precedence lower than .
Note the use of , entered as dd or \[DifferentialD], rather than ordinary d.
See The Mathematica Book: Section 1.10.4, Section 1.10.4 and Section 3.10.4.
See also: \[ContourIntegral] .