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Conditional signal assignment

The conditional signal assignment statement alters the projected waveform conditionally based on a boolean expression. The grammar rule is:

condition : expression(boolean)
conditional-waveforms : { waveform WHEN condition ELSE } waveform
conditional-signal-assignment : 
    target "<=" [options] conditional-waveforms ";"

This assignment consists of a list of waveforms that should be applied if the condition immediately following evaluates to true and a final waveform that is applied if no conditions are true. The conditions are checked sequentially from first to last and the first waveform of the first condition is applied to the projected waveform. If more than one condition is true, only the waveform corresponding to the first true condition is used. For example,

data<=data1 when sel=B"00" else 
      data2 when sel=B"01" else
      data3 when sel=B"10" else
      data4;

operates as a multiplexer of four inputs.