fn unary_infix(&mut self, f: fn(i64) -> i64) -> Result<()> {
        let a = self.pop()?;
        self.stack.push(f(a));
        Ok(())
    }
// infix operations that use a constant (and subsequently only pops once)
infix operations that use a constant (and subsequently only pops once)
infix operations that use a constant ( and subsequently only pops once )
fn unary_infix(&mut self, f: fn(i64) -> i64) -> Result<()> {
        let a = self.pop()?;
        self.stack.push(f(a));
        Ok(())
    }
fn unary_infix ( & mut self , f : fn ( i64 ) -> i64 ) -> Result < ( ) > { let a = self . pop ( ) ? ; self . stack . push ( f ( a ) ) ; Ok ( ( ) ) }
infix operations that use a constant (and subsequently only pops once)
infix operations that use a constant ( and subsequently only pops once )
&mut self
f
fn(i64) -> i64
&mut self
f
fn(i64) -> i64