// Relationals must be within parentheses
// Return arithmetic value 0 for false or 1 for true
//   <  <=  =  !=  >=  >
 i.e.: 
#declare BallColor = pigment { red (clock > 0.5) } // black or red

// Relational expressions: 
// (A < B)        A is less than B 
// (A <= B)       A is less than or equal to B 
// (A = B)        A is equal to B (actually abs(A-B)<EPSILON) 
// (A != B)       A is not equal to B (actually abs(A-B)>=EPSILON) 
// (A >= B)       A is greater than or equal to B 
// (A > B)        A is greater than B 

// As logical operators there are '&' and '|' standing for
// logical AND and OR.
// (A & B)        true only if both A and B are true, false otherwise 
// (A | B)        true if either A or B or both are true 
// They return arithmetic value 0 for false or 1 for true.
// Note that A and B are not handled bitwise but logical.

#if (A & B)
  ... // do this only if both A and B are true
#else // optional
  ... // do this only if both A and B are false
#elseif ( Cond ) // optional
  ... // do this only if both A and B are false and Cond true
#end

#if (A | B)
  ... // do this if either A or B is true
#end

#ifdef ( IDENTIFIER )
  ... // do this only if both A and B are true
#end

#ifndef ( IDENTIFIER )
  ... // do this only if both A and B are true
#end

#undef IDENTIFIER //