// 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 


// Logical expressions:
// (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 


// The conditional expression returns a value depending
// on whether an expression is true or false.
// i.e.:
#declare Value=((X>Y)?A:B);  // if X is larger than Y, A is returned, otherwise B.

// with an boolean value FLAG (0 or 1): 
#declare value=(FLAG  ? A : B);  // if FLAG = 1 then A, else B.