SWI-Prolog -- function atan2/2

Documentation
- Reference manual
  - Built-in Predicates
    - Arithmetic
      - General purpose arithmetic
        
        Arithmetic Functions
        
        -/1
        
        +/1
        
        +/2
        
        -/2
        
        */2
        
        //2
        
        mod/2
        
        rem/2
        
        ///2
        
        div/2
        
        rdiv/2
        
        gcd/2
        
        lcm/2
        
        abs/1
        
        sign/1
        
        copysign/2
        
        nexttoward/2
        
        roundtoward/2
        
        max/2
        
        min/2
        
        ./2
        
        random/1
        
        random_float/0
        
        round/1
        
        integer/1
        
        float/1
        
        rational/1
        
        rationalize/1
        
        numerator/1
        
        denominator/1
        
        float_fractional_part/1
        
        float_integer_part/1
        
        truncate/1
        
        floor/1
        
        ceiling/1
        
        ceil/1
        
        >>/2
        
        <</2
        
        \//2
        
        /\/2
        
        xor/2
        
        \/1
        
        sqrt/1
        
        sin/1
        
        cos/1
        
        tan/1
        
        asin/1
        
        acos/1
        
        atan/1
        
        atan2/2
        
        atan/2
        
        sinh/1
        
        cosh/1
        
        tanh/1
        
        asinh/1
        
        acosh/1
        
        atanh/1
        
        log/1
        
        log10/1
        
        exp/1
        
        **/2
        
        ^/2
        
        powm/3
        
        lgamma/1
        
        erf/1
        
        erfc/1
        
        pi/0
        
        e/0
        
        epsilon/0
        
        inf/0
        
        nan/0
        
        cputime/0
        
        eval/1
        
        msb/1
        
        lsb/1
        
        popcount/1
        
        getbit/2
- Packages

Availability:Arithmetic function (see is/2)

[ISO]atan2(+YExpr, +XExpr)

Result = arctan(YExpr/XExpr). Result is the angle in radians. The return value is in the range [- pi ... pi ]. Used to convert between rectangular and polar coordinate system.

Note that the ISO Prolog standard demands atan2(0.0,0.0) to raise an evaluation error, whereas the C99 and POSIX standards demand this to evaluate to 0.0. SWI-Prolog follows C99 and POSIX.