Software　design　is　a(46)　process.　It　requires　a　certain　amount　of　fl　air　on　the　part　of　the(47).　Design　can　not　be　learned　from　a　book　.　It　must　be　practiced　and　learnt　by　experience　and　study　of　existing　systems.　A　well(48)　software　system　is　straightforward　to　implement　and　maintain　,　easily(49)　and　reliable.　Badly(48)　software　systems,　although　they　may　work　are(50)　to　be　expensive　to　maintain　,　difficult　to　test　and　unreliable.