diff --git a/config/locales/en.yml b/config/locales/en.yml
index 73bc16a..cc40eb7 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -27,8 +27,8 @@ en:
               parent_domain_mismatch: 'parent quantity has to be in the same domain'
             formula:
               invalid_formula: 'interpretation failed: %{msg}'
-              disallowed_token: 'includes disallowed token "%{token}"
-                (of type %{ttype}) at %{location}'
+              unparsable_formula: 'cannot be parsed'
+              disallowed_token: 'includes disallowed token "%{token}" (of type: %{ttype})'
               disallowed_function_call: 'includes disallowed function call: %{function}'
               unknown_quantity: 'contains undefined quantity: %{quantity}'
   body_trackers:
diff --git a/lib/body_tracking/formula.rb b/lib/body_tracking/formula.rb
index 275c317..ac05b00 100644
--- a/lib/body_tracking/formula.rb
+++ b/lib/body_tracking/formula.rb
@@ -10,6 +10,58 @@ module BodyTracking
         @formula = formula
       end
 
+      #def validate
+      #  errors = []
+
+      #  # 1st: check if formula is valid Ruby code
+      #  #tokenized_length = Ripper.tokenize(@formula).join.length
+      #  #unless tokenized_length == @formula.length
+      #  #  errors << [:invalid_formula, {part: @formula[0...tokenized_length]}]
+      #  #end
+      #  begin
+      #    eval("-> { #{@formula} }")
+      #  rescue ScriptError => e
+      #    errors << [:invalid_formula, {msg: e.message}]
+      #  end
+
+      #  # 2nd: check if formula contains only allowed token types
+      #  # 3rd: check for disallowed function calls (they are not detected by Ripper.lex)
+      #  # FIXME: this is unreliable (?) detection of function calls, should be replaced
+      #  # with parsing Ripper.sexp if necessary
+      #  identifiers = []
+      #  disallowed_functions = Set.new
+      #  prev_ttype, prev_token = nil, nil
+      #  Ripper.lex(@formula).each do |location, ttype, token|
+      #    puts ttype, token
+      #    case
+      #    when QUANTITY_TTYPES.include?(prev_ttype) && ttype == :on_lparen
+      #      disallowed_functions << prev_token unless FUNCTIONS.include?(prev_token)
+      #      identifiers -= [prev_token]
+      #    when prev_ttype == :on_period && QUANTITY_TTYPES.include?(ttype)
+      #      disallowed_functions << token unless FUNCTIONS.include?(token)
+      #    when is_token_quantity?(ttype, token)
+      #      identifiers << token
+      #    when [:on_sp, :on_int, :on_rational, :on_float, :on_tstring_beg, :on_tstring_end,
+      #          :on_lparen, :on_rparen, :on_period].include?(ttype)
+      #    when :on_op == ttype &&
+      #      ['+', '-', '*', '/', '%', '**', '==', '!=', '>', '<', '>=', '<=', '<=>', '===',
+      #       '..', '...', '?:', 'and', 'or', 'not', '&&', '||', '!'].include?(token)
+      #    when :on_kw == ttype && ['and', 'or', 'not'].include?(token)
+      #    else
+      #      errors << [:disallowed_token, {token: token, ttype: ttype, location: location}]
+      #    end
+      #    prev_ttype, prev_token = ttype, token unless ttype == :on_sp
+      #  end
+      #  disallowed_functions.each { |f| errors << [:disallowed_function_call, {function: f}] }
+
+      #  # 4th: check if identifiers used in formula correspond to existing quantities
+      #  identifiers.uniq!
+      #  quantities = @project.quantities.where(name: identifiers).pluck(:name)
+      #  (identifiers - quantities).each { |q| errors << [:unknown_quantity, {quantity: q}] }
+
+      #  errors
+      #end
+
       def validate
         errors = []
 
@@ -28,30 +80,70 @@ module BodyTracking
         # 3rd: check for disallowed function calls (they are not detected by Ripper.lex)
         # FIXME: this is unreliable (?) detection of function calls, should be replaced
         # with parsing Ripper.sexp if necessary
+        # failing test vectors:
+        # - fcall disallowed: "abs(Fats)+Energy < 10"
+        # working test vectors:
+        #   a.abs(Fats)+Energy < 10
         identifiers = []
         disallowed_functions = Set.new
-        prev_ttype, prev_token = nil, nil
-        Ripper.lex(@formula).each do |location, ttype, token|
-          puts ttype, token
-          case
-          when QUANTITY_TTYPES.include?(prev_ttype) && ttype == :on_lparen
-            disallowed_functions << prev_token unless FUNCTIONS.include?(prev_token)
-            identifiers -= [prev_token]
-          when prev_ttype == :on_period && QUANTITY_TTYPES.include?(ttype)
-            disallowed_functions << token unless FUNCTIONS.include?(token)
-          when is_token_quantity?(ttype, token)
-            identifiers << token
-          when [:on_sp, :on_int, :on_rational, :on_float, :on_tstring_beg, :on_tstring_end,
-                :on_lparen, :on_rparen, :on_period].include?(ttype)
-          when :on_op == ttype &&
-            ['+', '-', '*', '/', '%', '**', '==', '!=', '>', '<', '>=', '<=', '<=>', '===',
-             '..', '...', '?:', 'and', 'or', 'not', '&&', '||', '!'].include?(token)
-          when :on_kw == ttype && ['and', 'or', 'not'].include?(token)
+
+        stree = [Ripper.sexp(@formula)]
+        errors << [:unparsable_formula, {}] unless stree.first
+
+        while stree.first
+          ttype, token, *rest = stree.shift
+          case ttype
+          when :program, :args_add_block
+            stree.unshift(*token)
+          when :binary
+            operator, token2 = rest
+            stree.unshift(token, token2)
+          when :method_add_arg
+            stree.unshift(token, *rest)
+          when :call
+            stree.unshift(token)
+            dot, method = rest
+            ftype, fname, floc = method
+            disallowed_functions << fname unless FUNCTIONS.include?(fname)
+          when :fcall
+            ftype, fname, floc = token
+            disallowed_functions << fname
+          when :vcall
+            ftype, fname, floc = token
+            identifiers << fname
+          when :arg_paren
+            stree.unshift(token)
+          when :var_ref
+            vtype, vname, vloc = token
+            identifiers << vname
+          when :@int
           else
-            errors << [:disallowed_token, {token: token, ttype: ttype, location: location}]
+            byebug
+            errors << [:disallowed_token, {token: token, ttype: ttype}]
           end
-          prev_ttype, prev_token = ttype, token unless ttype == :on_sp
         end
+
+        #Ripper.lex(@formula).each do |location, ttype, token|
+        #  puts ttype, token
+        #  case
+        #  when QUANTITY_TTYPES.include?(prev_ttype) && ttype == :on_lparen
+        #    disallowed_functions << prev_token unless FUNCTIONS.include?(prev_token)
+        #    identifiers -= [prev_token]
+        #  when prev_ttype == :on_period && QUANTITY_TTYPES.include?(ttype)
+        #    disallowed_functions << token unless FUNCTIONS.include?(token)
+        #  when is_token_quantity?(ttype, token)
+        #    identifiers << token
+        #  when [:on_sp, :on_int, :on_rational, :on_float, :on_tstring_beg, :on_tstring_end,
+        #        :on_lparen, :on_rparen, :on_period].include?(ttype)
+        #  when :on_op == ttype &&
+        #    ['+', '-', '*', '/', '%', '**', '==', '!=', '>', '<', '>=', '<=', '<=>', '===',
+        #     '..', '...', '?:', 'and', 'or', 'not', '&&', '||', '!'].include?(token)
+        #  when :on_kw == ttype && ['and', 'or', 'not'].include?(token)
+        #  else
+        #    errors << [:disallowed_token, {token: token, ttype: ttype, location: location}]
+        #  end
+        #  prev_ttype, prev_token = ttype, token unless ttype == :on_sp
+        #end
         disallowed_functions.each { |f| errors << [:disallowed_function_call, {function: f}] }
 
         # 4th: check if identifiers used in formula correspond to existing quantities
@@ -81,7 +173,8 @@ module BodyTracking
         inputs.map do |i, values|
           begin
             [i, [get_binding(values).eval(paramed_formula), nil]]
-          rescue Exception
+          rescue Exception => e
+            puts e.message
             [i, [nil, nil]]
           end
         end