diff --git a/doc/cli-design.txt b/doc/cli-design.txt
index 15e0016..119b5b9 100644
--- a/doc/cli-design.txt
+++ b/doc/cli-design.txt
@@ -2,7 +2,7 @@ A flexible music programming language powered by math.
 
 usage:
 
-cmd [FILENAME | -c SHEET_MUSIC | --syntax | --examples | --example <n>] [-e EXPR] -n NOTE[,...] [-E [-f {"stdout" | FORMAT}]] [-v CHAR=VALUE]
+cmd [FILENAME | -c SHEET_MUSIC | --syntax | --examples | --example <n>] [-e EXPR] [-l EXPR] -n NOTE[,...] [-E [-f {"stdout" | FORMAT}]] [-v CHAR=VALUE]
 
 arguments:
     FILENAME        file path to your .blp sheet music  [default: read from stdin]
@@ -14,7 +14,9 @@ options:
         -n NOTE[,...], --notes NOTE[,...]
             available notes: a list of comma-separated alphabetical note names
         -e EXPR, --expression EXPR
-            signal expression (instrument) to generate music samples (you can use the floating-point number t for the current time in seconds, n for the indice of the currently playing note in the list of available notes starting with 0 and N for the number of available notes)  [default: sin(2*PI*(442+442*((n+1)/N))*t)]
+            signal expression (instrument) to generate music samples (you can use the floating-point number t for the current time in seconds, n for the indice of the currently playing note in the list of available notes starting with 0 and N for the number of available notes)  [default: "sin(2*PI*(442+442*((n+1)/N))*t)"]
+        -l EXPR, --length EXPR
+            expression to generate note length in seconds  [default: "2^(2-log_2(l))*(60/T)"]
 
     exporting music:
         -E, --export
@@ -24,7 +26,7 @@ options:
 
     advanced sheet music tweaking:
         -v VARIABLE=VALUE, --variable VARIABLE=VALUE
-            add a variable named VARIABLE (an single letter) for the sheet music and all expressions and set its initial value to VALUE (you can override n, t and T)
+            add a variable named VARIABLE (an single letter) for the sheet music and all expressions and set its initial value to VALUE (you can override n, t, l and T which is the only way to set the initial tempo)
         -m NAME:EXPANSION, --macro MACRO:EXPANSION
             add a macro named NAME (single character, must not be alphanumeric) which expands to EXPANSION when called in the sheet music
         -s NAME:VARIABLE=EXPR, --slope VARIABLE=EXPR