Tweak fibonacci example

Author: mkantor

examples/.snapshot

@@ -3,7 +3,7 @@ exports[`examples > fibonacci.plz --input="not a number" > stderr 1`] = `
 `;
 
 exports[`examples > fibonacci.plz --input="not a number" > stdout 1`] = `
-"input must be a natural number"
+"--input must be a natural number"
 
 `;
 
@@ -12,7 +12,7 @@ exports[`examples > fibonacci.plz --input=-1 > stderr 1`] = `
 `;
 
 exports[`examples > fibonacci.plz --input=-1 > stdout 1`] = `
-"input must be a natural number"
+"--input must be a natural number"
 
 `;
 
@@ -57,7 +57,7 @@ exports[`examples > fibonacci.plz > stderr 1`] = `
 `;
 
 exports[`examples > fibonacci.plz > stdout 1`] = `
-"missing input argument"
+"missing --input argument"
 
 `;
 

examples/fibonacci.plz

@@ -1,21 +1,22 @@
 {
   fibonacci: n =>
     @if {
-      condition: :n < 2
+      :n < 2
       then: :n
       else: :fibonacci(:n - 1) + :fibonacci(:n - 2)
     }
 
   input: @runtime { context =>
+    // read --input from command-line arguments
     :context.arguments.lookup(input)
   }
 
   output: :input match {
-    none: _ => "missing input argument"
+    none: _ => "missing --input argument"
     some: input => @if {
-      condition: :natural_number.is(:input)
+      :natural_number.is(:input)
       then: :fibonacci(:input)
-      else: "input must be a natural number"
+      else: "--input must be a natural number"
     }
   }
 }.output