Accidental use can turn out to be so frequent, however, that the original perform drops out of memory. The useful necessities that define most design issues don’t prescribe explicitly what should be optimized; often they set levels to be attained minimally. It is then up to the engineer to choose on how far to go beyond meeting the necessities on this minimal sense.Efficiency, in energy consumption and use of supplies to start with, is then usually a main value. Under the pressure of society, other values have come to be included, in particularsafety and, more recently, sustainability.

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