Let us focus on the step idea. It is necessary to distinguish between code statements and steps. Most of the Alvis statements e.g. exec, exit, etc. are single-step statements. On the other hand, if, loop and select are multi-step statements. We use recursion to count the number of steps for multi-step statements. For each of them, the first step enters the statement interior. Then, we count steps of statements put inside curly brackets.

For a given statement s, let stepno(s) denote the number of the step related to s. For multi-step statements, stepno(s) denotes the number of the step connected with entering the statement interior.

Let us consider the piece of code shown in Listing 1. The steps number are put inside comments. For example, the step 7 denotes entering the if statement, while the step 8 denotes the out statement.

agent A {
  i :: Int = 0;
  loop {                            -- 1
    select {                        -- 2
      alt (i == 0) { 
        in p;                       -- 3 
        i = 1;                      -- 4
      }  
      alt (i == 1) { 
        in q;                       -- 5
        i = 0;                      -- 6
      }
    }
    if(i == 1) {                    -- 7
      out p;                        -- 8
    }         
    else { 
      null;                         -- 9
    }  
  }
}  

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