pub fn f(&self) -> Result<f64, SexpError> {
        let s = self.string()?;
        let f = s.parse()?;
        Ok(f)
    }
/// access the symbolic-expression as if it is a String
/// that is a f64
access the symbolic-expression as if it is a String
that is a f64
access the symbolic - expression as if it is a String that is a f64
pub fn f(&self) -> Result<f64, SexpError> {
        let s = self.string()?;
        let f = s.parse()?;
        Ok(f)
    }
pub fn f ( & self ) -> Result < f64 , SexpError > { let s = self . string ( ) ? ; let f = s . parse ( ) ? ; Ok ( f ) }
access the symbolic-expression as if it is a String
that is a f64
access the symbolic - expression as if it is a String that is a f64
&self