source/js/animation/TL.Animate.js
function by(val, start, m, r, i) {
    return (m = relVal.exec(val)) ?
      (i = parseFloat(m[2])) && (start + (m[1] == '+' ? 1 : -1) * i) :
      parseFloat(val)
  }
function by(val, start, m, r, i) {
    return (m = relVal.exec(val)) ?
      (i = parseFloat(m[2])) && (start + (m[1] == '+' ? 1 : -1) * i) :
      parseFloat(val)
  }
function by ( val , start , m , r , i ) { return ( m = relVal . exec ( val ) ) ? ( i = parseFloat ( m [ 2 ] ) ) && ( start + ( m [ 1 ] == '+' ? 1 : - 1 ) * i ) : parseFloat ( val ) }
support for relative movement via '+=n' or '-=n'
support for relative movement via + = n or - = n